Prof. B. Thilaganathan, Fetal Medicine Unit, St George's University of London, Cranmer Terrace, London SW17 0RE, UK (e-mail: firstname.lastname@example.org)
Monochorionic (MC) pregnancies are routinely delivered electively at late preterm gestation with the aim of avoiding stillbirth at term. The aim of this study was to evaluate the prospective risk of late stillbirth in a large regional cohort of twin pregnancies of known chorionicity.
This was a retrospective study of all twin pregnancy births of known chorionicity between 2000 and 2009 from a large regional cohort consisting of nine hospitals. Prospective risk was calculated per 1000 fetuses rather than pregnancies, as each twin pregnancy had two gestations at risk of stillbirth.
A total of 3005 twin pregnancies delivered after 26 weeks' gestation in the Southwest Thames Obstetric Research Collaborative. The total risk of stillbirth after 26 weeks in MC twins (19.1 per 1000 fetuses) was significantly higher than in dichorionic (DC) twins (6.5 per 1000 fetuses), with an odds ratio (OR) of 2.97 (95% CI, 1.71–5.18). The risk of stillbirth in MC twins did not change significantly between 26 weeks (1.8 per 1000 fetuses) and 36 weeks (3.4 per 1000 fetuses), with an OR of 1.85 (95% CI, 0.3–13.2). The equivalent figures for DC twins were 0.6 per 1000 fetuses and 2.1 per 1000 fetuses, respectively (OR, 3.4 (95% CI, 0.9–13.2)).
The risk of perinatal loss in monochorionic (MC) twin pregnancies is variably estimated to be two to five-fold higher than in dichorionic (DC) twin pregnancies1–4. The increased risk of stillbirth is mainly attributed to complications of placental and vascular sharing in an MC placenta resulting in either fetal growth restriction or fetal transfusion syndromes5. Furthermore, in the event of an MC twin death, there is a high risk of death or neurodevelopmental handicap in the cotwin, which is thought to be a consequence of acute fetal exsanguination of the surviving cotwin through still patent placental vascular anastomoses6.
The increased risk of twin stillbirth is not only confined to early or mid pregnancy, but higher rates of MC twin death have been shown even after 32 weeks' gestation7. However, despite the increased risk of late stillbirth, the relatively low prevalence of MC twins has resulted in a paucity of epidemiological evidence on which to base clinical decisions about the optimal timing of MC twin birth to avoid intrauterine fetal death. As the correct modality and frequency of fetal surveillance to prevent stillbirth at term are still not evident, clinicians inevitably deliver MC pregnancies electively at various late preterm gestations. It is uncertain whether this practice is effective in preventing MC stillbirth. However, the practice of late preterm delivery is known to significantly increase the risk of neonatal morbidity and mortality8.
The aim of this study was to evaluate the prospective risk of late stillbirth in a large regional cohort of twin pregnancies of known chorionicity.
Patients and Methods
This was a retrospective cohort study of data collected by nine hospitals in the Southwest Thames region of England between 2000 and 2009. All women registering for routine antenatal care by 11 weeks' gestation with a confirmed diamniotic twin pregnancy were considered suitable for inclusion. Scan data were obtained by a computerized search of each hospital's obstetric ultrasound computer database, while birth details were obtained from their computerized maternity records. These two databases were cross-checked to ensure full data capture of twin pregnancies during the study period. Pregnancies of unknown chorionicity, delivery prior to 26 weeks' gestation, termination of pregnancy and stillbirth with a birth weight of < 500 g were exclusion criteria.
Gestational age was determined by measurement of the crown–rump length of the larger twin at the routine 11–14 weeks' scan or, in case of in-vitro fertilization, the date of embryo transfer9. Chorionicity was determined by first-trimester ultrasound assessment according to the presence or absence of the lambda/T-signs10. A routine fetal structural survey was carried out at 20–22 weeks and fetal growth and wellbeing were assessed serially every 3 to 5 weeks from 28 weeks onwards, or more frequently as clinically indicated. All MC twins had two additional scans, at 17 and 19 weeks, specifically to identify early features of twin–twin transfusion syndrome (TTTS). If TTTS was suspected, the mother was referred to the local tertiary center for fetoscopic laser ablation of the placental interconnecting vessels11. Decisions regarding mode of delivery were made according to individual patient's wants and circumstances and the attending obstetrician's own clinical practice. For planned vaginal birth, induction of labor was offered from 38 weeks' gestation, and elective Cesarean sections were scheduled from 36 weeks for MC twins and from 37 weeks for DC twins.
Information on all stillbirths in the Southwest Thames region from 2000 to 2009 was obtained from the national registry. During the study period, all perinatal deaths after 22 weeks' gestation in the UK were required to be registered with the Centre for Maternal and Child Enquiries (CMACE). Data on stillbirths were linked and verified against the combined ultrasound and maternity databases. In accordance with CMACE regulations, patient identifiers such as name, hospital number and date of birth were not made available to the researchers. Individual stillbirth notifications were scrutinized to confirm the gestational age at which intrauterine death had been diagnosed and gestational age at delivery; if the former was not available, gestational age at birth was used as a proxy. This timing was based on the finding of an average 2-day time interval between fetal death and delivery in the third trimester and used for eight pregnancies in this cohort12. The chorionicity of all stillbirths was confirmed by placental histopathology or discordant neonatal sex. Ethical approval for this retrospective study was obtained from the local research ethics committee, and the study followed STROBE (strengthening the reporting of observational studies in epidemiology) guidelines for observational studies.
The risk of stillbirth was derived for each 2-week gestational-age interval from 26 weeks' gestation. It was calculated as the total number of stillbirths that occurred during the 2-week block divided by the number of ongoing fetuses at the beginning of the time period13. The number of ongoing fetuses was used as the denominator because it is a better representation of the risk of stillbirth in multiple pregnancies as, of course, for each pregnancy there are two fetuses at risk (the stillbirth risk for ongoing pregnancies is shown in Table S1 and Figure S1).
The data were analyzed using MS/Office Excel (Microsoft Corp., Redmond, WA, USA), Statsdirect (www.statsdirect.com) and CIA 1.1, 1991 (Martin Gardner, BMJ publication, London, UK) statistical packages.
In the 10-year study period, a total of 3081 twin pregnancies were identified through cross-linking the regional ultrasound and maternity databases. There were 76 exclusions, 13 because of lack of determination of chorionicity and 63 because of delivery before 26 weeks' gestation or stillbirths with a birth weight of < 500 g. Data on 3005 diamniotic twin pregnancies (549 MC and 2456 DC) delivering after 26 weeks' gestation were available and were included in the analysis.
The gestation, birth weight and birth-weight centile at delivery in the MC and DC twin pregnancies for both the live births and stillbirths are shown in Table 1. These parameters were statistically significantly different between MC and DC twins only in the pregnancies resulting in live births (Table 1 and Figure 1). The number of stillbirths and prospective risk of stillbirth for each 2-week period from 26 weeks' gestation are shown in Table 2 and Figure 2. The total risk of stillbirth after 26 weeks in MC twins (19.1 per 1000 fetuses) was significantly higher than in DC twins (6.5 per 1000 fetuses) (odds ratio (OR), 2.97 (95% CI, 1.71–5.18)). The risk of stillbirth in MC twins did not change significantly between 26 weeks (1.8 per 1000 fetuses) and 36 weeks (3.4 per 1000 fetuses) (OR, 1.85 (95% CI, 0.3–13.2)). The equivalent figures for DC twins were 0.6 per 1000 fetuses and 2.1 per 1000 fetuses, respectively (OR, 3.4 (95% CI, 0.9–13.2)).
Table 1. Neonatal data for monochorionic (MC) and dichorionic (DC) twin pregnancies
MC pregnancies (n = 549)
DC pregnancies (n = 2456)
Data are given as n, median (interquartile range) or mean ± SD.
Gestational age at delivery (weeks)
Birth weight (g)
2238 ± 551
2438 ± 599
Gestational age at death (weeks)
Birth weight (g)
1414 ± 596
1624 ± 612
Table 2. Prospective risk of stillbirth for monochorionic and dichorionic twins calculated per 1000 ongoing fetuses in 2-weekly blocks from 26 weeks' gestation
Gestational age (weeks)
Ongoing fetuses (n)
Fetal losses (n)
Risk of stillbirth (‰(95% CI))
Odds ratio (95% CI)
Ongoing fetuses (n)
Fetal losses (n)
Risk of stillbirth (‰(95% CI))
Odds ratio (95% CI)
26 + 0 to 27 + 6
28 + 0 to 29 + 6
30 + 0 to 31 + 6
32 + 0 to 33 + 6
34 + 0 to 35 + 6
> 36 + 0
There is an ongoing debate on the optimal timing for birth in otherwise uncomplicated MC twins. Clinical opinion is evenly divided between late preterm elective delivery to prevent late stillbirth3, 7, 14, 15 and expectant management as for DC twins16–19. The data of the current study demonstrate that the total stillbirth rate after 26 weeks' gestation in MC twins was approximately three times higher than in DC twins, but the prospective risk of stillbirth does not increase significantly after 26 weeks' gestation in either MC or DC twins.
The Southwest Thames Obstetric Research Collaborative cohort evaluated in this study is, to date, the largest twin pregnancy cohort of known chorionicity to be validated against both a concomitant delivery database and national stillbirth register. This cohort was managed in a manner consistent with current clinical practice and, importantly, had a modal time of delivery for MC and DC twins of 36 and 37 weeks' gestation, respectively. The risk of stillbirth in DC twins after 26 weeks remained static at around 1 per 1000 ongoing fetuses in each 2-week epoch, with a total stillbirth rate of 6.5 per 1000 ongoing fetuses. The two previous studies reporting the highest risk of MC stillbirth were characterized by no standardized protocol for fetal surveillance7 and elective delivery of MC twins at 37–38 weeks' gestation3. Therefore, the variation in stillbirth risk found in the literature is possibly explained by the wide variety of protocols for pregnancy monitoring, timing of elective birth and the retrospective nature of most studies. The discrepancy between the current and previous studies may be explained by the finding that in the current cohort, the modal time of delivery of MC twins (36 weeks) was before the expected rise in the risk of stillbirth. The risk of stillbirth in MC twins after 26 weeks remained static at around 3 per 1000 ongoing fetuses in each 2-week epoch, with a total stillbirth rate of 19 per 1000 ongoing fetuses. As for DC twins, it is possible that the level of antenatal care provided and modal time of delivery of 36 weeks' gestation caused suppression of a tendency for the stillbirth risk to increase near term. Even though the risk of stillbirth does not rise after 26 weeks' gestation, the total stillbirth rate after 26 weeks is about three times higher in MC than in DC twin pregnancies.
The data presented in this study do not appear to justify elective birth before 36 weeks' gestation in MC twins on the basis of an increased risk of stillbirth. However, it is evident that the total stillbirth rate after 26 weeks' gestation approaches 2%. An important consideration when deciding on the timing of birth of MC twins is the consequence of cotwin death. The best estimates suggest that up to 20% of survivors of cotwin death will have long-term neurological damage6. Given these circumstances, it would appear entirely reasonable to consider delivery of MC twins before 36 weeks' gestation. However, such a decision to effect late preterm birth in any pregnancy must be balanced against the potential harms of such a birth. Although the risk of undesirable neonatal outcomes decreases with increasing gestational age, the risk of neonatal complications in late preterm births remains higher than in infants delivered at 37–40 weeks' gestation8. Furthermore, the infant death rate after 32 weeks' gestation is less than 1%, the risk of respiratory distress syndrome at 32 weeks is about 5% and the likelihood of developing cerebral palsy is three times higher at 34 weeks than at term20, 21.
The main strength of this study is that it is the largest twin cohort to provide data on chorionicity-related risk of late stillbirth. A major limitation is the retrospective design of the study. However, validation of the ultrasound database against the delivery suite and national stillbirth registers allowed for capture of all pregnancy outcomes, as for a prospective study, and should reduce the impact of this limitation. The assumptions made about the gestational age at which intrauterine death was diagnosed in our study could have an impact on our findings.
In summary, the prospective risk estimates of stillbirth in MC and DC twins are given for a large, well characterized regional cohort. It is evident that the risk of stillbirth for MC twins does not increase significantly near term. Although this observation may be due to a policy of routine surveillance and elective delivery from 36 weeks' gestation, this finding does not support a policy of elective birth before 36 weeks in MC pregnancies.
SUPPORTING INFORMATION ON THE INTERNET
The following supporting information may be found in the online version of this article:
Table S1 Prospective risk of stillbirth for monochorionic and dichorionic twins per 1000 continuing pregnancies in the STORK dataset. Odds ratios are given for risk of stillbirth at each epoch compared to risk at 26 weeks' gestation. 95% confidence intervals (CI) are shown in parentheses. GA, gestational age.
Figure S1 Gestational age-specific risk of stillbirth expressed per 1000 continuing pregnancies in monochorionic () and dichorionic () twins in the STORK dataset.
The authors would like to acknowledge the support given by the members of the Centre for Maternal and Child Enquiries and the fetal medicine staff members of the referring hospitals in the Southwest Thames region.
Tiran Dias, St George's University of London
Dimitri Patel, St George's University of London
Amar Bhide, St George's University of London
Rosol Hamid, Mayday University Hospital
Hina Gandhi, East Surrey Hospital
Cheryl Ellis, Epsom General Hospital
Anne Deans, Frimley Park Hospital
Ifat Atullah, Kingston Hospital
Elisabeth Peregrine, Kingston Hospital
Matthew Jolly, Portsmouth Hospital
Renata Hutt, Royal Surrey Hospital
Aris Papageorghiou, St George's University of London
Peter Knott, St Helier's Hospital
Arash Bahamie, St Peter's Hospital
Faz Pakarian, Worthing Hospital
Basky Thilaganathan, St George's University of London