Womb Twin Survivors Have Lower Birth Weights

Are singleton survivors of a vanishing twin pregnancy more likely to have lower birth weights than other singletons? This question was the focus of a study from August 2017 published in Human Reproduction. The study was a Norwegian population-based registry study. The mothers who participated in the study all delivered between January 1984 and December 2013. All babies were conceived with fertility assistance. 17,291 singletons, 638 singletons with Vanishing Twin Syndrome and 2418 singletons with uncertain vanishing twin status were included in the study. Vanishing Twin Syndrome is when a twin or multiple disappears during pregnancy as a result of a miscarriage. Unlike other miscarriages, there are generally no signs of Vanishing Twin Syndrome. The miscarriage usually happens early in the pregnancy, in the first trimester.

Increased risk of being born small

The researchers concluded that Vanishing Twin Syndrome was associated with lower birth weight. This means that a remaining twin is more likely to be born with a lower birth compared to  singletons without a vanishing twin. The association between a vanishing twin and low birth weight in the survivor twin did not seem to be related to characteristics of the mother. The remaining twin also had an increased risk of being born small for his or her gestational age. The study also indicated that babies with a vanishing twin were more likely to be born preterm, even though this result wasn’t statistically significant.

The authors of this study were supported in part by the UK Medical Research Council, US National Institute of Environmental Health Sciences and the Norwegian Research Council.