What Causes Women to Have Twins

Sometimes, giving birth to twins causes complications for the babies and mother. One would think that the bearing and the raising of twins would take a heavy toll on a mother’s health and her children, but a new study found that the opposite is true. The study showed that women who have twins have healthier and heavier single babies before and after giving birth to twins. Other studies have found that twinning is a sign of maternal health; it is associated with greater fertility and longevity of the mother. An explanation for these findings is that twinning mothers possess higher levels of IGF-1 (insulin growth factor-1). IGF-1 is a protein that can stimulate the ovaries to release more than one egg and it can also stimulate the growth of the fetus. Cattle with high levels of IGF-1 are known to give birth to twins.

Bigger babies

Some women tend to have more often twins and this tendency may run in families. One hypothesis proposes that twinning is caused by high levels of IGF-1 in women. Because IGF-1 is known to increase the growth of the fetus, researchers analyzed a huge amount of data, collected in rural Africa, to compare the birth weight of single babies by twinning mothers to the birth weight of babies by non-twinning mothers. Most of the women gave birth to large numbers of children. They found that single babies born to mothers, who had twins before, were substantially larger than babies born to mothers, who never had twins. Even more surprisingly, single babies born to mothers before they had twins were also larger than the average babies born to mothers who never had twins. The authors of the report conclude that the IGF-1 level in the mother is likely responsible for twinning and that twinning provides an evolutionary survival advantage.

Health benefits of twinning

The causes of twinning, whether IGF-1 or other yet unknown factors, may not only provide benefits to single babies born to twinning mothers, but may benefit the health of the mother. A study in Utah analyzed comprehensive data from 58,789 non-polygamous women, who were born between 1807 and 1899. Researchers did not distinguish between fraternal and identical twins, but identical twins are rarer. The study found that women bearing twins were healthier, more fertile, and lived longer than women who never had twins.


Both genes and nutrition influence the IGF-1 status of women. Note that the benefits of twinning were only associated with naturally occurring twinning. Twins or multiple births, produced by artificial or hormonal means, may not provide health benefits to babies or mothers.


Rickard, I.J et al. Twinning propensity and offspring in utero growth covary in rural African women. Biology Letters (2011) doi: 10.1098/rsbl.2011.0598





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