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Twin birth defect risk may be higher among mothers not on fertility treatment

Received wisdom is that fertility treatment might explain higher abnormality rate among twins

Caroline White

Tuesday, 21 June 2016

The risk of birth defects among twins may be higher among mums who haven’t used fertility treatment—which is known to increase the chances of a twin birth—than among those who have used it, finds US research* published online in the Journal of Epidemiology & Community Health.

The incidence of twin births is on the rise, and it is thought that fertility treatment may help explain this increase. But it is not clear whether the treatment might itself be linked to the higher rate of birth defects seen among twins, as previous studies have not always included information on its use.

In an effort to clarify the associations between twins and their higher rate of birth defects, the researchers mined data from the National Birth Defects Prevention Study (NBDPS)—a major population based, case control study of major birth abnormalities in the USA.

They included mothers of singletons and twins with no major birth defects (comparison group), born between October 1997 and December 2007, as well as cases of major birth abnormalities drawn from stillbirths, pregnancy terminations, and live births during the same period.

Just under 3% (227) of the mothers in the comparison group had twins compared with just under 6% (1,250) of the mothers in the birth defect group.

In both groups, mothers of twins were more likely to be older than 29, of white ethnicity, to be more highly educated, and to have taken folic acid before and around pregnancy than the mothers of singletons.

Use of fertility treatment was also more common among twin pregnancies than it was among singleton pregnancies both for mothers in the comparison group (32.5% vs just over 3%, respectively) and for mothers in the birth defect group (29% vs 4%, respectively).

Among the mothers who used fertility treatment, twinning was associated with five out of 25 different types of birth defect, with the strongest associations found for hypoplastic left heart syndrome, an abnormality that affects normal blood flow through the heart, and omphalocele (a hernia that pushes the abdominal organs into the umbilical cord).

But among the mothers who didn’t use fertility treatment, twinning was associated with 29 out of 45 different types of birth defect, with the strongest associations seen for defects in the wall dividing the left and right chambers of the heart and an abnormality in which the abdominal organs are exposed.

The risk of birth defects was higher among identical than among non-identical twins.

This is an observational study so no firm conclusions can be drawn about cause and effect, and the researchers caution that the numbers of mothers in the group not using fertility treatment was small, added to which, around 30% of eligible mothers didn’t take part in the NBDPS study.

“Explanations for the observed associations between twinning and birth defects are not clear, although our results suggest that there may be differences depending on use of fertility treatment, zygosity of twins, and the birth defect being considered,” they write.

They speculate that the reasons for the associations found might lie in a common cause, particularly among identical twins, or in the consequences of twinning itself, which might include crowding, insufficient nutrient supply, and problems with blood flow between the two foetuses.

Less than optimal fertility, associated with obesity or older age, may also have a role, they suggest.

* Dawson AL, et al. Twinning and major birth defects, National Birth Defects Prevention Study, 1997-2007. J Epidemiol Community Health doi:10.1136/jech-2015-206302

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