Twins who have shared the womb with a sister are healthier at birth than those who shared with a brother. Newborn twins with a sister are less likely to be pre-term, have a low birth weight, and have fewer respiratory problems, a new study suggests.
The research found that among 2704 twin pregnancies, the risk of a pre-term delivery was highest if the twins were both male, with nine per cent being born before the 31st week of pregnancy. Seven and a half per cent of mixed-sex twins, and just five and a half per cent of female twins were born before the 31st week.
In all, 70 per cent of the study group were mixed-sex twins, 14 per cent were male/male, and 16 per cent were male/female. All of the twins were non-identical to ensure the full effect of the fetuses' sex could be seen.
'A male fetus affects his co-twin negatively, probably due to the inter-fetal transport of substances, mainly hormones,' said co-author of the study, Dr Marek Glezerman, who is a professor at the Sackler School of Medicine at Tel Aviv University in Israel, and chairman of the Helen Schneider Hospital for Women's obstetrics and gynecology department.
However, the researchers are warning women expecting male or mixed-sex twins not to panic, since they see these higher risks of birth problems as demonstrative of a male 'disadvantage' rather than a female 'protective factor'. Previous research has already shown that boys are more likely than girls to be born prematurely, need a caesarean or have health problems at delivery. Experts have suggested that in addition to the negative effect of certain hormones, there is a possibility that males compete better for nutrients because females grow more slowly in the womb.
In this study, females who shared with males were more likely to be born with neurological problems or underdeveloped lungs than those who shared with another female. There were also differences in birth weight between the twins in this study. Boys with a male twin had lower birth weights than twin boys with a sister, at 4.85 pounds on average compared with 4.99 pounds.
Both Dr Abdulla Al-Khan, director of perinatal diagnostics and therapeutics in the maternal-fetal medicine division at Hackensack University Medical Center in New Jersey, US, and Dr Steven Ory, a past president of the American Society for Reproductive Medicine and a fertility specialist in Margate, Florida, US have pointed out some of the study's limitations. They believe it was open to bias because of its retrospective nature, and furthermore, the babies' method of conception was not disclosed.