Women in their 30s are more likely to conceive fraternal twins as an evolutionary “insurance policy” to compensate for declining fertility and combat an increased risk of early fetal loss, new research suggests.
Fraternal twins, or dizygotic twins, are those who developed from separate fertilized eggs and are genetically distinct but were carried in the womb at the same time. Monozygotic twins, better known as identical twins, result from the fertilization of one egg that later splits into two, causing the two individuals to share the same genes and sex, according to the National Human Genome Research Institute. Dizygotic twinning has been described as an “evolutionary paradox” – it’s long been known that older women are more likely to simultaneously release two eggs in their cycle, but the evolutionary strategy behind it has remained largely unknown. Previous research has suggested that older women are more likely to conceive twins, but not necessarily carry each to term. Researchers now say that as biological females age, she is more likely to double ovulate – that is, when two eggs are released during the same month of a menstrual cycle – to increase the likelihood of carrying at least one fertilized egg to term.
To explain the likelihood of women carrying twins in their mid-life, an international team of researchers simulated various scenarios using postnatal and maternal survival rates for twins and single children from women who naturally conceived. They then compared the expected lifetime reproductive success of women with respect to age and their likelihood of double ovulating.
“We resolve this paradox by showing that twinning and non-twinning are not competing strategies; instead, dizygotic twinning is the outcome of an adaptive conditional ovulatory strategy of switching from single to double ovulation with increasing age,” write the authors in Nature Ecology & Evolution. “This conditional strategy, when coupled with the well-known decline in fertility as women age, maximizes reproductive success and explains the increase and subsequent decrease in the twinning rate with maternal age that is observed across human populations.”
At least 90 percent of twinning rates were explained by the shift from single to double ovulating as a woman aged, but with this also comes an increased risk of fetal loss. The researchers add that this “increased rate of double ovulation does not translate into ever-increasing rates of twin births.” The frequency of double ovulation evolved so that biological females could maintain fertility.
“Our work suggests that the probability of releasing two eggs increases as women age due to an evolved response to the age-related decline in embryo viability,” said Associate Professor Joseph Tomkins, from The University of Western Australia School of Biological Sciences, in a statement.
“Using simulated tests, we were able to show that in ancestral populations, evolution would favor a scenario whereby two eggs are released but only a single child is born, suggesting fraternal twin births are a by-product of selection for fertility, rather than a way to increase reproductive output.”
The researchers conclude that their work contributes to a broader understanding of population growth as many families around the world are choosing to delay having children.