Change in smoking status during two consecutive pregnancies: a population-based cohort study

Authors


  • This work was conducted without any financial or other support or assistance from any tobacco company or individual or entity acting on behalf of the tobacco industry. This work has not been previously published nor is it being considered for publication elsewhere.

Abstract

Objective

To investigate changes in tobacco smoking in two consecutive pregnancies and factors associated with the change.

Design

Population-based cohort study.

Setting

New South Wales, Australia, 2000–10.

Population

A total of 183 385 women having first and second singleton pregnancies.

Methods

Descriptive and multivariable logistic regression analyses of perinatal data linked to hospital admission data.

Main outcome measures

Proportion of women smoking during their first pregnancy who quit by their second, and of women not smoking in their first pregnancy who did smoke during their second.

Results

Among 22 761 smokers in the first pregnancy, 33.5% had quit by their second. Among 160 624 non-smokers in their first pregnancy, 3.6% smoked during their second. Women who were aged ≥25 years, were married, born in a non-English speaking country, used private obstetric care, and lived in a socio-economically advantaged area were more likely to quit or less likely to start smoking in the second pregnancy. Smokers who had gestational hypertension (adjusted odds ratio [OR] 1.36, 95% confidence interval [95% CI] 1.23–1.51), a large-for-gestational-age infant (OR 1.66, 95% CI, 1.46–1.89), and a stillbirth (OR 1.44, 95% CI 1.06–1.94) were more likely to quit, whereas smokers whose infant was small-for-gestational-age (OR 0.65, 95% CI 0.60–0.70) or admitted to special care nursery (OR 0.87, 95% CI 0.81–0.94) were less likely to quit. Among non-smokers in the first pregnancy, the risk of smoking in the second pregnancy increased with late antenatal attendance (e.g. ≥26 weeks, OR 1.30, 95% CI 1.14–1.48), gestational diabetes (OR 1.25, 95% CI 1.07–1.45), preterm birth (e.g. spontaneous, OR 1.25, 95% CI 1.10–1.43), caesarean section (e.g. prelabour, OR 1.13, 95% CI 1.01–1.26), and infant small-for-gestational-age (OR 1.37, 95% CI 1.26–1.48) or required special care nursery (OR 1.14, 95% CI 1.06–1.23). Inter-pregnancy interval of ≥3 years was associated with either change in smoking status.

Conclusions

Most smokers continue to smoke in their next pregnancy, even among those who experienced poor outcomes. Intensive interventions should be explored and offered to women at the highest risk.

Introduction

The prevalence of tobacco smoking during pregnancy remains high in high-income western countries (11–17%),[1-5] despite a decline over the past two decades.[1, 3, 6] Smoking during pregnancy is one of the most potentially preventable causes of a range of adverse pregnancy outcomes, including placental abruption, miscarriage, fetal growth restriction, preterm births and perinatal deaths.[6-9] Quitting smoking reduces the risk of preterm births and low birthweight by 20%.[9] However, in western countries, less than half of women (4–47%) quit spontaneously when they become pregnant.[10] A majority of smoking women (69–75%) continue to smoke in their subsequent pregnancies.[4, 8, 11-13] Existing studies from the USA,[11, 12] Sweden[4] and Norway[13] report that maternal sociodemographic circumstances and psychological distress play important roles in maternal smoking in subsequent pregnancies. However, there is a paucity of evidence on whether experience of an adverse pregnancy outcome in an index pregnancy influences maternal smoking status during the next pregnancy.[4, 11]

Australia is a world-leader in tobacco control policies.[14] Public health measures, including increased taxation, smoke-free environment legislation, expanded graphic health warnings, restricted tobacco advertising, and plain packaging and display have reduced the smoking rate in the general population.[14] There are concerted efforts among health professionals across disciplines to promote smoking cessation in pregnancy.[7, 15, 16] Clinical practice guidelines to assist health professionals in identifying pregnant women who smoke and providing them with interventions and support to quit have been developed and disseminated broadly.[7, 15, 16] As a result, increasing levels of advice and support for smoking cessation have been offered to pregnant smoking women.[17, 18] In addition, in Australia, eligible smokers are provided with access to subsidised smoking cessation medications including nicotine replacement therapy, varenicline and bupropion.[15] Currently, an evaluation of the use, effectiveness and safety of smoking pharmacotherapies among Australian pregnant women is underway.[19]

Within Australia's context of well-developed tobacco control policies and publicly funded health system, the extent to which women change their smoking status between pregnancies and—importantly—factors that influence this behavioural change, are unknown. Identifying these determinants can inform strategies and interventions to better target women at high risk of smoking during pregnancy. This study investigated the extent to which women who smoked during their first pregnancy quit by their second, and women who did not smoke in their first pregnancy did smoke during their second, and how the change in smoking status varied according to first pregnancy factors, including maternal sociodemographic characteristics, medical conditions, antenatal attendance and pregnancy outcomes.

Methods

Data sources and linkage

The study cohort included 183 385 women in New South Wales (NSW), Australia, who had first and second singleton pregnancies between July 2000 and December 2010. NSW is Australia's most populous state with 7.2 million residents as of June 2011[20] and 93 000 births per annum delivered in more than 100 hospitals.[21] The study used perinatal records extracted from the NSW Perinatal Data Collection (referred to as ‘perinatal record’) and maternal hospital admission records (for a delivery) extracted from the NSW Admitted Patient Data Collection (referred to as ‘hospital admission record’). Perinatal records were linked to hospital admission records to create a set of longitudinal and enriched data about the cohort's consecutive pregnancies.

The Perinatal Data Collection is a legislated and population-based surveillance system covering all births (live births and stillbirths of at least 20 weeks of gestation or at least 400 g birthweight) in NSW public and private hospitals, and at home. The perinatal data collect information on the health of mothers and babies, including maternal sociodemographic characteristics, smoking status, pre-existing and gestational diabetes and hypertension, and outcomes of the pregnancy.[21]

The Admitted Patient Data Collection is a mandatory data collection of all hospital discharges from all public, private and repatriation hospitals, and private day procedure centres in NSW. The data contain maternal sociodemographic characteristics, diagnoses and procedures undertaken. For each hospital admission record, between one and 55 diagnoses are coded according to the International Statistical Classification of Diseases and Related Health Problems, Tenth Revision, Australian Modification (ICD-10-AM).[22]

Records from the two sources of data were linked by the NSW Centre for Health Record Linkage. As Australia does not have a unique registration number for citizens, the linkage process used a probabilistic matching method[23, 24] and the privacy preserving approach.[25] The validity of the probabilistic record linkage is extremely high, with <0.3% false-positive links, and <0.5% missed links.[23, 24] Only de-identified data were given to researchers.

Study outcomes and variables

The primary outcome of the study was the proportion of women who smoked during their first pregnancy and quit by their second pregnancy, and conversely, the proportion of women who did not smoke in the first pregnancy but smoked during their second pregnancy.

The perinatal data included a check box (‘Yes’, ‘No’) for the item ‘Did the mother smoke cigarettes at all during pregnancy?’.[21] In hospital admission data, the assignment of the Z72.0 diagnostic code in any diagnosis field indicates the use of tobacco in the last month.[22] For this study, a woman was identified as a smoker if this was indicated in the perinatal record or in the hospital admission record. The recording of maternal smoking in perinatal records and hospital admission records has been validated previously (medical records as the gold standard), with respective sensitivities of 89.6% and 66.2%, respective positive predictive values of 96.1% and 93.0%, and specificities of 99.0%.[26, 27] Our algorithm to combine smoking information from the two sources of data has similar utility to using medical records in identifying pregnant smokers.[28] A total of 846 women (0.5%) with an unknown smoking status in either the first (n = 395) or second pregnancy (n = 469) were excluded.

Change in smoking status between pregnancies was examined according to sociodemographic and clinical characteristics. Maternal sociodemographic characteristics, including age, country of birth, indigenous status, marital status and remoteness and socio-economic status of residential areas were revealed from the perinatal record for the first pregnancy, whereas the use of private or public obstetric care was ascertained from the hospital admission record. Country of birth was grouped as English-speaking (Australia, New Zealand, UK and North America), Asia (South East, North East and Southern Asia), and other non-English speaking countries.[29] Remoteness of residential area was based on the extended version of the Accessibility/Remoteness Index of Australia,[30] while socio-economic status of residential area was based on quintiles of the Index of Relative Socio-economic Disadvantage.[31]

Maternal medical conditions during the first pregnancy, including pre-existing diabetes mellitus (diabetes), gestational diabetes and respiratory conditions, were identified from the hospital admission record, whereas hypertensive disorders (pre-existing or gestational hypertension) were detected from perinatal record or hospital admission record. Gestational age (weeks) at the initial antenatal attendance (in the first pregnancy) was obtained from the perinatal record. In Australia, it is recommended that the initial antenatal assessment ideally be completed by 12 weeks of gestation.[32] Inter-pregnancy interval was calculated as the number of years between the first infant's date of birth and the estimated conception date of the second pregnancy. Women with inter-pregnancy interval less than 6 months (n = 27) were further excluded from analyses because they were likely to carry different risk profiles.[33]

Outcomes of the first pregnancy derived from the perinatal record included preterm birth (<37 weeks, medically indicated or spontaneous), mode of delivery (spontaneous vaginal, instrumental, prelabour and intrapartum caesarean section), stillbirth, infant birthweight (small-for-gestational-age [SGA] <10th birthweight centile; large-for-gestational-age >90th centile; and appropriate-for-gestational-age), Apgar score at 5 minute (<7), and baby admission to special care nursery or neonatal intensive care unit (referred to as ‘special care’). Maternal postpartum haemorrhage was identified from the hospital admission record. Existing evidence[26, 27, 34] indicates high levels of agreement between medical records and perinatal records or hospital admission records in relation to the recording of these medical diagnoses and obstetric information.

Data analysis

Analyses were stratified by maternal smoking status during the first pregnancy. We used frequency distribution and contingency analyses to describe the patterns of change in smoking status from the first to second pregnancy. Multivariable logistic regression models were used to assess the association between the change in smoking status and maternal sociodemographic characteristics, medical conditions, antenatal attendance, and outcomes of the first pregnancy. Odds ratios (OR) and 95% confidence intervals (95% CI) were adjusted for maternal age, country of birth, marital status, indigenous status, private obstetric care, residential remoteness and socio-economic status, diabetes, hypertensive disorder, respiratory conditions, antenatal attendance, year of first delivery, and inter-pregnancy interval. All analyses were carried out using SAS version 9.3 (SAS Institute, Cary, NC, USA).[35]

Results

Maternal sociodemographic and clinical characteristics of the first pregnancy are presented in Table 1. Among 22 761 women who smoked during the first pregnancy, 7632 (33.5%) had quit by their second pregnancy. Among 160 624 women who did not smoke in the first pregnancy, 5829 (3.6%) did smoke in the second pregnancy. On average, smokers who had quit were 2.3 years older (mean age 25.6 years) than those who did not quit (mean age 23.3 years). Women not smoking in the first pregnancy but who smoked during the second were 4.0 years younger (mean age 24.7 years) than those who did not smoke in either pregnancy (mean age 28.7 years). Mean inter-pregnancy interval was 2.7 years (standard deviation 1.5), regardless of maternal smoking status. Seventy-seven (38.1%) smokers whose first baby was stillborn had quit by the second pregnancy (Table 1).

Table 1. Sociodemographic and clinical characteristics of the first pregnancy according to maternal smoking status, n (column %)
 Smoked during first pregnancyDid not smoke in first pregnancy
Quit by second pregnancy, n (%)Smoked during second pregnancy, n (%)Did not smoke in second pregnancy, n (%)Smoked during second pregnancy, n (%)
  1. a

    Among live births.

Total number 7632 (100.0)15 129 (100.0)154 795 (100.0)154 795 (100.0)
Maternal age (years)
<201350 (17.7)4812 (31.8)7365 (4.8)1458 (25.0)
20–242416 (31.7)5524 (36.5)27 099 (17.5)1791 (30.7)
25–292114 (27.7)2953 (19.5)56 193 (36.3)1436 (24.6)
30–341355 (17.8)1434 (9.5)49 205 (31.8)881 (15.1)
≥35397 (5.2)406 (2.7)14 933 (9.6)263 (4.5)
Country of birth
English-speaking6978 (91.4)14 533 (96.1)120 031 (77.5)5126 (87.9)
Asian209 (2.7)113 (0.7)19 394 (12.5)267 (4.6)
Other non-English speaking445 (5.8)483 (3.2)15 370 (9.9)436 (7.5)
Marital status
Never married2910 (38.1)8154 (53.9)18 895 (12.2)2261 (38.8)
Widow, separated, others147 (1.9)361 (2.4)2291 (1.5)117 (2.0)
Married, de-facto4575 (59.9)6614 (43.7)133 609 (86.3)3451 (59.2)
Private/public obstetric care
Private1205 (15.8)845 (5.6)63 462 (41.0)920 (15.8)
Public6427 (84.2)14284 (94.4)91 333 (59.0)4909 (84.2)
Remoteness
Urban area4482 (58.7)7233 (47.8)111 790 (72.2)3742 (64.2)
Rural area3143 (41.2)7889 (52.1)42 850 (27.7)2081 (35.7)
Unknown7 (0.1)7 (0.0)155 (0.1)6 (0.1)
Socio-economic status
1st quintile (lowest)1375 (18.0)3474 (23.0)21 389 (13.8)1294 (22.2)
2nd quintile1572 (20.6)3894 (25.7)23 209 (15.0)1289 (22.1)
3rd quintile2038 (26.7)4300 (28.4)33 179 (21.4)1391 (23.9)
4th quintile1491 (19.5)2358 (15.6)33 455 (21.6)1120 (19.2)
5th quintile (highest)1152 (15.1)1101 (7.3)43 538 (28.1)735 (12.6)
Unknown4 (0.1)2 (0.0)25 (0.0)0 (0.0)
Initial antenatal attendance (weeks)
1–124429 (58.0)8717 (57.6)103 712 (67.0)3241 (55.6)
13–252742 (35.9)5261 (34.8)45 663 (29.5)2212 (37.9)
≥26390 (5.1)971 (6.4)4626 (3.0)311 (5.3)
Unknown71 (0.9)180 (1.2)794 (0.5)65 (1.1)
Diabetes mellitus
No diabetes7412 (97.1)14 725 (97.3)148 225 (95.8)5615 (96.3)
Pre-existing diabetes23 (0.3)52 (0.3)496 (0.3)18 (0.3)
Gestational diabetes197 (2.6)352 (2.3)6074 (3.9)196 (3.4)
Hypertensive disorders
No hypertension6797 (89.1)13 890 (91.8)136 014 (87.9)5188 (89.0)
Pre-existing hypertension74 (1.0)99 (0.7)1852 (1.2)42 (0.7)
Gestational hypertension761 (9.9)1140 (7.5)16 929 (10.9)599 (10.3)
Respiratory conditions
No7468 (97.9)14 731 (97.4)153 067 (98.9)5743 (98.5)
Yes164 (2.1)398 (2.6)1728 (1.1)86 (1.5)
Mode of delivery
Vaginal4523 (59.3)10 004 (66.1)80 110 (51.8)3530 (60.6)
Instrumental1256 (16.5)2041 (13.5)32 798 (21.2)928 (15.9)
Prelabour caesarean546 (7.2)902 (6.0)14 279 (9.2)404 (6.9)
Intrapartum caesarean1305 (17.1)2171 (14.4)27 513 (17.8)965 (16.6)
Unknown2 (0.0)11 (0.1)95 (0.1)2 (0.0)
Postpartum haemorrhage
No7102 (93.1)14 233 (94.1)143 276 (92.6)5410 (92.8)
Yes530 (6.9)896 (5.9)11 519 (7.4)419 (7.2)
Gestation age (weeks)
≥377097 (93.0)13 886 (91.8)146 208 (94.5)5399 (92.6)
<37, medically indicated184 (2.4)372 (2.5)3321 (2.1)166 (2.8)
<37, spontaneous350 (4.6)871 (5.8)5263 (3.4)264 (4.5)
Unknown1 (0.0)0 (0.0)3 (0.0)0 (0.0)
Stillbirth/Live birth
Live birth7554 (99.0)14 995 (99.1)153 839 (99.4)5780 (99.2)
Stillbirth77 (1.0)125 (0.8)898 (0.6)44 (0.8)
Unknown1 (0.0)9 (0.1)58 (0.0)5 (0.1)
Apgar 5 minute a
<7157 (2.1)279 (1.9)2444 (1.6)107 (1.9)
≥77391 (97.8)14 687 (97.9)151 278 (98.3)5662 (98.0)
Unknown6 (0.1)29 (0.2)117 (0.1)11 (0.2)
Birthweight for gestational age a
Appropriate5929 (78.5)11 102 (74.0)125 180 (81.4)4572 (79.0)
Small1092 (14.5)3300 (22.0)15 652 (10.2)757 (13.1)
Large528 (7.0)584 (3.9)12 854 (8.4)446 (7.7)
Unknown5 (0.1)9 (0.1)153 (0.1)5 (0.1)
Special care nursery admission a
No6227 (82.4)12 087 (80.6)129 452 (84.1)4757 (82.3)
Yes1327 (17.6)2908 (19.4)24 387 (15.9)1023 (17.7)

Results of multivariable logistic regression analyses are shown in Table 2. Over the study period (2000–09), there was an increase in likelihood of pregnant smokers quitting by the second pregnancy. Smokers who were aged ≥25 years were married, were born in an Asian or another non-English speaking country, used private obstetric care, or lived in a socio-economically advantaged area (4th and 5th socio-economic status quintiles) in the first pregnancy were significantly more likely to quit than other smokers. Women having these sociodemographic characteristics and not smoking in the first pregnancy were significantly less likely to start smoking in the second pregnancy than other non-smoking women. Inter-pregnancy interval of 3 years or longer was associated with both a greater likelihood of quitting (OR 1.51, 95% CI 1.40–1.62) and a greater risk of starting to smoke in the second pregnancy (OR 1.34, 95% CI 1.25–1.43) (Table 2).

Table 2. Factors associated with change in smoking status between pregnancies: adjusted OR and 95% CI
 Quitting smoking by second pregnancy among smokers during first pregnancySmoking during second pregnancy among non-smokers during first pregnancy
%OR (95% CI)a%OR (95% CI)a
  1. a

    Among live births.

  2. **Adjusted for maternal age, country of birth, Indigenous status, marital status, private obstetric care, socio-economic status, remoteness, diabetes, hypertension, antenatal attendance, year of delivery, and birth interval.

Year of first delivery
200031.61.04.71.0
200133.71.08 (0.95–1.24)4.50.98 (0.87–1.10)
200233.51.09 (0.96–1.24)3.80.85 (0.75–0.96)
200333.21.11 (0.97–1.27)3.50.80 (0.71–0.90)
200433.31.13 (0.98–1.29)3.40.77 (0.68–0.88)
200533.91.21 (1.06–1.39)3.40.84 (0.74–0.95)
200634.91.28 (1.11–1.48)3.20.80 (0.70–0.91)
200734.11.30 (1.12–1.51)3.61.00 (0.88–1.13)
200834.01.42 (1.20–1.68)2.90.83 (0.72–0.97)
200930.21.37 (1.07–1.75)3.20.77 (0.61–0.99)
Birth interval (years)
<229.31.03.31.0
2–2.935.31.25 (1.16–1.35)3.11.01 (0.95–1.08)
≥337.01.51 (1.40–1.62)4.91.34 (1.25–1.43)
Maternal age (years)
<2021.91.016.51.0
20–2430.41.28 (1.19–1.39)6.20.51 (0.47–0.56)
25–2941.71.67 (1.52–1.83)2.50.30 (0.27–0.33)
30–3448.61.85 (1.66–2.06)1.80.26 (0.23–0.29)
≥3549.41.84 (1.56–2.17)1.70.26 (0.23–0.30)
Country of birth
English speaking32.41.04.11.0
Asian64.92.89 (2.27–3.67)1.40.30 (0.27–0.35)
Other non-English speaking48.01.43 (1.24–1.64)2.80.57 (0.51–0.63)
Marital status
Never married26.31.010.71.0
Widow, separated, others28.90.92 (0.75–1.13)4.90.73 (0.60–0.89)
Married, de-facto40.91.35 (1.27–1.44)2.50.53 (0.49–0.56)
Private/public obstetric care
Public31.01.05.11.0
Private58.81.91 (1.72–2.11)1.40.50 (0.46–0.55)
Remoteness
Rural area28.51.04.61.0
Urban area38.31.21 (1.14–1.29)3.21.19 (1.12–1.27)
Socio-economic status
1st quintile (lowest)28.41.05.71.0
2nd quintile28.81.00 (0.92–1.10)5.30.95 (0.87–1.03)
3rd quintile32.21.11 (1.02–1.21)4.00.82 (0.76–0.89)
4th quintile38.71.26 (1.15–1.39)3.20.83 (0.76–0.90)
5th quintile (highest)51.11.63 (1.46–1.82)1.70.55 (0.50–0.61)
Initial antenatal attendance (weeks)
1–1233.71.03.01.0
13–2534.31.09 (1.02–1.16)4.61.21 (1.14–1.28)
≥2628.71.01 (0.88–1.15)6.31.30 (1.14–1.48)
Diabetes mellitus
No diabetes33.51.03.61.0
Pre-existing diabetes30.70.74 (0.44–1.24)3.51.09 (0.68–1.71)
Gestational diabetes35.90.83 (0.69–1.00)3.11.25 (1.07–1.45)
Hypertensive disorders
No hypertension32.91.03.71.0
Pre-existing hypertension42.81.28 (0.93–1.76)2.20.61 (0.45–0.83)
Gestational hypertension40.01.36 (1.23–1.51)3.40.81 (0.74–0.89)
Respiratory disorders
No33.61.03.61.0
Yes29.20.92 (0.76–1.11)4.70.90 (0.72–1.13)
Mode of delivery
Vaginal31.11.04.21.0
Instrumental38.11.08 (1.00–1.18)2.80.97 (0.90–1.05)
Prelabour caesarean37.70.93 (0.82–1.05)2.81.13 (1.01–1.26)
Intrapartum caesarean37.51.09 (1.01–1.18)3.41.14 (1.06–1.23)
Postpartum haemorrhage
No33.31.03.61.0
Yes37.21.13 (1.01–1.27)3.50.90 (0.81–1.00)
Gestational age (weeks)
≥3733.81.03.61.0
<37, medically indicated33.10.85 (0.70–1.03)4.81.49 (1.26–1.77)
<37, spontaneous28.70.90 (0.79–1.03)4.81.25 (1.10–1.43)
Stillbirth 
No33.51.03.61.0
Yes38.11.44 (1.06–1.94)4.71.28 (0.93–1.76)
Apgar 5 minute a
≥733.51.03.61.0
<736.01.16 (0.94–1.42)4.21.05 (0.86–1.28)
Birthweight for gestational age
Appropriate34.81.03.51.0
Small24.90.65 (0.60–0.70)4.61.37 (1.26–1.48)
Large47.51.66 (1.46–1.89)3.40.91 (0.83–1.01)
Special care nursery admission a
No34.01.03.51.0
Yes31.30.87 (0.81–0.94)4.01.14 (1.06–1.23)

Smokers who had gestational hypertension (OR 1.36, 95% CI 1.23–1.51), stillbirth (OR 1.44, 95% CI 1.06–1.94), or large-for-gestational-age infant (OR 1.66, 95% CI 1.46–1.89) were more likely to quit by their second pregnancy, whereas those who had the first infant born SGA (OR 0.65, 95% CI 0.60–0.70) or admitted to the special care (OR 0.87, 95% CI 0.81–0.94) were less likely to quit. Among women who did not smoke in the first pregnancy, factors associated with smoking during the second pregnancy included a late antenatal attendance (≥13 weeks of gestation), gestational diabetes, preterm birth, caesarean section, SGA infant, or the first baby admitted to the special care nursery (Table 2).

Discussion

Main findings

In this population-based prospective study, 33.5% of women who smoked during the first pregnancy had quit by their second pregnancy, and 3.6% of women who were not reported as smokers in the first pregnancy reported smoking during the second pregnancy. These changes in maternal smoking status between pregnancies varied, to a larger extent by maternal sociodemographic characteristics (age, country of birth, marital status, use of private care and residential area) and inter-pregnancy interval, and to a lesser extent by maternal medical conditions and outcomes of the first pregnancy.

Strengths and limitations

The main advantage of the current study was the use of whole-of-population data collected over a 10-year period. Linking perinatal data to hospital admission data allowed not only greater identification of pregnant smokers[28] but also the examination of various determinants of maternal smoking across pregnancies. Very few studies have investigated the role of maternal medical diagnoses, mode of delivery, the timing of the initial antenatal attendance, and adverse outcomes of the index pregnancy. As such, this study provides a greater level of evidence about the effects of clinical factors on maternal smoking in the subsequent pregnancy.

Nevertheless, some limitations need to be considered. Maternal smoking status was primarily self-reported. There was no information on contextual risk factors for smoking and barriers to smoking cessation, including household income, maternal educational attainment, social support, household smoking environment and exposure to passive smoking.[1, 3, 6, 36, 37] Additionally, it was not possible to investigate the role of co-existing maternal depression and mental illnesses because these conditions are likely to be under-reported in hospital admission data.[38] Early pregnancy loss such as miscarriage and termination of pregnancy before 20 weeks of gestation are not included in the perinatal data, and amount of tobacco consumption is not consistently recorded throughout the pregnancy.[26] As a result, these determinants could also not be examined in this study. The timing of changes in maternal smoking status was unknown. Finally, the generalisability of the study findings to other populations in settings with less developed tobacco control policies may be limited.

Interpretation

The proportion of smokers in this study who quit by the second pregnancy (33.5%) was higher than those being reported elsewhere (24.9–30.9%).[4, 11-13] The proportion of women who started smoking during the second pregnancy (3.6%) was lower than those being reported in US studies (4.8–6.2%)[11, 12] but higher than the finding from a Norwegian study (2.3%).[13] It is an encouraging finding that the trend in quitting increased over the study period, which is consistent with a decline in prevalence of smoking in the general female population and in the overall obstetric population in Australia during the study period.[28] These findings are likely to reflect the effectiveness of Australia's tobacco-control public health interventions,[14] and the efforts of the health system to promote smoking cessation among pregnant women.[17, 18]

This study found a significant role of country of birth, in which smokers born in a country other than Australia, New Zealand, UK or North America were more likely to quit or never smoke in either pregnancy. It has been reported that Asia-born female immigrants who had longer duration of residence in Australia were more likely to smoke cigarettes, but compared with Australia-born women, they were less likely to be daily or heavy smokers.[39] Given the low female smoking rates in Asia, Africa and the Middle East,[14] it is possible that women migrating from these regions are likely to quit spontaneously during pregnancy, or are likely to be responsive to smoking cessation advice.

A previous study has shown that women who smoked during consecutive pregnancies have significantly higher risk of preterm birth and low birthweight in the later pregnancy in comparison to smokers who quit.[8] It is, therefore, of concern that in this study, two-thirds of women who smoked during the first pregnancy continued to do so during the next pregnancy despite adverse outcomes (stillborn, SGA and special care admission). It would seem intuitive that women who had an adverse outcome might be inclined to stop smoking to protect the second baby. Our study findings reflect the highly addictive nature of tobacco smoking and strongly reinforce the need for appropriate interventions and support for both nulliparous women, and those who previously had adverse pregnancy outcomes. Counselling for smoking women who experienced pregnancy loss,[32] clearly, has not been completely effective to reduce smoking in these women.

The implementation of the 5As protocol approach (Ask, Advise, Assess, Assist and Arrange for follow up)[7, 15, 16, 40] for smoking cessation has demonstrated certain improvement in identifying women who smoke during pregnancy, and delivery of interventions.[17, 18, 41] However, existing evidence[17, 18, 41, 42] suggests that the provision of interventions is still suboptimal. In Australia, about half of pregnant smokers were not given advice, written information, or referrals to smoking cessation services, and were not asked about smoking status in subsequent visits.[17, 42] Similarly, in the USA, only 42% of pregnant smokers were asked, then given advice and assistance to quit during the first trimester.[41] Perinatal care is often shared among obstetricians, midwives, general practitioners and other health professionals in a variety of healthcare settings, which may interfere with the identification of smokers and the provision of smoking cessation interventions. Further strategies are needed to identify smoking women and implement timely intervention. Smoking cessation pharmacotherapies and hypnotherapy are reported as preferred methods for quitting by pregnant smokers.[42]

Consistent with previous studies,[4, 11, 13] this study found that the risk of change in maternal smoking increases with longer inter-pregnancy interval. Among smokers, longer inter-pregnancy interval is likely to reflect successful attempts to quit before the next pregnancy, but among non-smokers, it would indicate greater likelihood of exposure to stressors associated with smoking. It has been reported that socio-economically disadvantaged women often report parenting challenges, greater incidence of disruptive home or neighbourhood environment, and feelings of isolation and lack of social support, so often view smoking as a solution to relieve their emotional issues.[36] Both the immediate post-birth and preconception periods, certainly, should be explored as potential opportunities for interventions. Particular attention is needed for disadvantaged groups to ensure their access to smoking cessation programmes, given the strong association between sociodemographic factors and the increased risk of smoking in the second pregnancy.

It was another concerned finding that a change from non-smoking to smoking was greater among women who had experienced an adverse outcome (gestational diabetes, preterm birth, SGA infant, caesarean section and baby requiring special care). This finding might reflect an increase in disclosure of smoking in the second pregnancy, rather than starting smoking among these women. Non-disclosure of smoking among Australian pregnant smokers is relative low (9%)[37] compared with those in some other countries, such as Scotland (22–39%),[43] USA (22.9%)[44] and New Zealand (25%).[45] Women who did not disclose their smoking in the first pregnancy may be more likely to disclose in the second pregnancy if they perceive themselves to be at higher risk of an adverse outcome. The clarity of the questions being asked about smoking, encouragement from clinical staff and motivations to quit are likely to contribute to voluntary self-reporting of smoking behaviours among pregnant women.[7, 36, 43] Alternatively, women may be aware of the health consequences of tobacco smoking during pregnancy, it is unknown whether women with the first infant SGA would think smoking might make the second delivery easier if the baby is small.

Conclusion

Despite harmful effects of smoking during pregnancy, two-thirds of pregnant smokers continued to smoke during the next pregnancy. General public tobacco-control measures and antenatal smoking cessation interventions are not uniformly effective for all women of child-bearing age. Further health system supports are needed to assist maternity-care providers to identify women at risk so that smoking cessation advice and other interventions can be implemented before, during and after pregnancy. Target intervention strategies should also be explored, especially for women who are at higher risk of smoking.

Disclosure of interests

All authors have no conflict of interests.

Contribution to authorship

All authors have contributed to the study and approved the final version of this manuscript. DTT designed the study, planned and executed analysis and drafted the manuscript. CR and LJ supervised the conception, design and analysis, and assisted with manuscript preparation. AH and SS contributed to the design and assisted with manuscript preparation.

Details of ethics approval

Ethics approval for the study has been granted by the NSW Population and Health Services Research Ethics Committee (Reference number: HREC/12/CIPHS/44 on 24 September 2012).

Funding

This work was conducted as part of The Smoking MUMS study, which is funded by a National Health and Medical Research Council Project Grant (#1028543). CR is supported by a National Health and Medical Research Council Senior Research Fellowship (#1021025). AH is supported by a National Health and Medical Research Council Early Career Fellowship (#1013287).

Acknowledgement

We thank the NSW Ministry of Health for access to the population health data and the NSW Centre for Health Record Linkage for linking the data sets.

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