Distinguishing monozygotic (MZ; identical) from dizygotic (DZ; non-identical) twins is important to parents of twins. It is often the first question that parents of twins are asked by family and friends. Parents also use knowledge about their twins’ zygosity to understand similarities and differences in physical appearance and behaviour. Determining the zygosity of same-sex twin pairs is not always easy, so we were interested to find out how parents of twins obtained zygosity information.
Data were from the Gemini study, a birth cohort of 2402 families with twins born in 2007 in England or Wales.1 Parents of same-sex twins (n = 1586) were asked if they knew whether their twins were MZ or DZ, and if so, how they had discovered. The majority of parents (n = 1302, 82%) said that they had been given zygosity information by health professionals based on the separateness of placentation as seen on prenatal scans, these are the twin pairs included in the present analysis. For the remaining twin pairs the opinion of the health professional was based on other information (n = 22, e.g. DNA, septal thickness), or on information that was not specified or not known by the parents (n = 113), or conflicting information was given to parents (n = 13), or no opinion had been given (n = 136), so these were not included in this analysis.
As part of the routine Gemini data collection, all parents in this sample (n = 1302) had completed a validated zyposity questionnaire,2 when twins were on average 8.2 months old (SD = 2.2, range 4–10 months). We confirmed these results with DNA analysis in 81 randomly selected pairs; obtaining 100% agreement with the questionnaire allocation. Based on the zygosity questionnaire (and available DNA data), 651 twin pairs were classified as MZ, 621 as DZ and 30 pairs could not be classified because of conflicting answers in the questionnaire and unavailable DNA (see Table 1).
|Questionnaire/DNA-based zygosity||Reported health professional prediction and reason|
(n = 489)
(separate placentas and sacs)
(n = 813)
|MZ (n = 651)||72.5% (n = 472)||27.5% (n = 179)|
|DZ (n = 621)||1.9% (n = 12)||98.1% (n = 609)|
|Uncertain (n = 30)||16.7% (n = 5)||83.3% (n = 25)|
We found that 191 of the 1302 parents (14.7%) were misinformed about zygosity based on prenatal scan observations. As many as 27.5% of parents of MZ twins mistakenly believed that their twins were DZ (i.e. 179 out of 651); compared with only 2% of parents of DZ twins who mistakenly believed their twins were MZ (i.e. 12 out of 621) (see Table 1). In total, 38% of parents said that they were told after an antenatal scan that their twins shared a placenta and were therefore MZ (n = 489), whereas 62% of parents were told that their twins were DZ because they were dichorionic diamniotic (DCDA) (n = 813).
In fact, 25–30% of MZ twins are DCDA because if the MZ zygote splits within 2 days of fertilisation, separate placentas develop.3,4. However, a 2004 survey among members of the American College of Obstetricians and Gynecologists reported a lack of knowledge regarding chorionicity and zygosity, with 81% of physicians believing that dichorionic twins are always DZ.5 Our data suggest that there may be a similar lack of knowledge among some health professionals in the UK, because the proportion of parents who appeared to have been misinformed closely matched the prevalence of DCDA in MZ twins.
The separateness of the placentas and amniotic sacs in utero can be determined during prenatal ultrasound scans and parents are often informed by health professionals about the significance of such observations for zygosity. However the primary purpose of these ultrasound scans is not to predict zygosity, they are clinically important in assessing the risk associated with monochorionic twin pregnancies.6 It is a common misconception that these observations can be used to conclusively and accurately classify twin zygosity. Two separate placentas and amniotic sacs do not denote dizygosity, because if MZ twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from that of DZ twins.
Most parents seemed convinced that observations from antenatal scans provided conclusive results about zygosity; for example one parent wrote ‘I know for certain that my twins are non-identical as they had separate placentas and sacs.’ Another wrote ‘Antenatal scans showed they were DCDA, so clearly non-identical, also they look different to me but this would be influenced by having been told they are fraternal.’ We do not know whether they had been informed of the 25% chance that DCDA same-sex twins are identical, but the lack of uncertainty in their responses, suggested that they had not been. After we disclosed zygosity information to Gemini families who had indicated that they wanted zygosity feedback, one family wrote: ‘We felt angry and let down by the health professionals, who even after they were born, told us they could not be identical due to their separate sacs and placentas. I felt confused, as if I did not really know my children or what my body had done.’ However, this study is limited because all data were based on parent-recalled information on what health professionals had said to them. Objective data on the actual information that was provided to parents was lacking.
Should health professionals satisfy parents’ curiosity by advising them about zygosity based on scan observations? We believe that parents have a right to zygosity information if it is available; they take it seriously and value it in terms of their twins’ identity. Most parents are likely to be able to understand a probabilistic result if it is explained appropriately. The 12-week ultrasound scan can be used to examine the placental tissue at the base of the intertwin membrane for accurate prediction of chorionicity.6,7,8 If the placental tissue extends into the base of the intertwin membrane (the ‘T sign’), it is extremely likely that the twins are monochorionic, and parents can be told that the twins are over 95% likely to be identical. On the other hand, finding a ‘lamda’ sign or ‘twin peak’ provides strong evidence for dichorionicity, and parents can be told that there is a 3-in-4 chance that their twins are non-identical and a 1-in-4 chance that they are identical, if they are of the same sex. These results indicate that some additional professional training may be needed to ensure that misconceptions about the relationship between placentation and zygosity are dispelled.