A general perception exists that places the risk for birth defects higher in babies conceived via in vitro fertilization (IVF) and other forms of assisted reproductive technologies (ARTs) than for babies conceived naturally. Two studies on twins and birth defects, recently published by various members of the same research team, have effectively debunked that myth. The studies revealed who’s most likely to have twins and that naturally conceived twins have a higher risk for birth defects although these defects are not the same kind ART-conceived twins are prone to experience.
The Research Team
April Dawson, who is lead author for both studies, is a medical researcher at the US Centers for Disease Control and Prevention’s (CDC’s) National Center on Birth Defects and Developmental Disabilities. Dr. Jennita Reefhuis, also of the CDC, was senior author. The rest of the research team included additional researchers from the CDC as well as the Massachusetts Center for Birth Defects Research and Prevention, MassGeneral Hospital for Children, the National Human Genome Research Institute, the Oak Ridge Institute for Science and Education, and the University of Arkansas College of Medicine. Other than Dawson and Reefhuis, some of the researchers only worked on one study or the other. Both studies used data from the National Birth Defects Prevention Study, 1997 – 2007.
Who Has Twins
This study compared live singleton and twin births, some of whom had orofacial clefts (cleft lip and/or cleft palate). The control group (without orofacial clefts) was comprised of:
- 228 twins and
- 8,242 singletons.
The case group (with orofacial clefts) included:
- 117 twins and
- 2,859 singletons.
In both groups (with and without orofacial clefts), the mothers of naturally conceived twins were more likely to be non-Hispanic black women and/or tobacco smokers. Factors that increased the chance of twins conceived of IVF or other ARTs were older maternal age, higher income, and state of residence (insurance coverage for ART varies by state).
Conception Method Influence on Twins’ Birth Defects
Researchers used the same database for this study but included all birth defects, not just orofacial clefts.
Natural Conception
When conception occurred naturally, there were 45 types of birth defects between singletons and twins. Twins were found to be at much greater risk for 29 of these 45 defect types than singletons.
The most common birth defects in this group were multiple ventricular septal defects (a hole in a wall of the heart) and cloacal exstrophy (internal organs protruding outside the abdominal cavity at birth; genital defects may also occur and the anus may not have an opening).
ART Conception
There were only 25 types of defects found in babies conceived via ART, with five types closely associated with twins.
Most common defects in this group were hypoplastic left heart syndrome (the left side of the heart is poorly developed and may interfere with blood flow) and omphalocele (this defect of the muscles of the abdominal wall often causes the liver and/or intestines to be positioned outside the abdominal cavity).
Zygosity may influence birth defects in twins but the birth defects database did not include this information. Fraternal twins (dizygotic, produced from two eggs) are easily identified when one baby is male and the other female but, without DNA testing, fraternal twins of the same sex may appear identical (monozygotic, produced from one egg).
Sources:
Dawson, April L, et al. "Epidemiology of twinning in the National Birth Defects Prevention Study, 1997–2007."PMC. Birth Defects Research Part A: Clinical and Molecular Teratology / John Wiley & Sons Ltd., 31 Oct. 2014. US National Library of Medicine / National Institutes of Health. Web. 8 July 2016.
Dawson, April L, et al. "Twinning and major birth defects, National Birth Defects Prevention Study, 1997–2007."Journal of Epidemiology & Community Health / BMJ Publishing Group Ltd. doi:10.1136/jech-2015-206302 (2016). BMJ. Web. 8 July 2016.