The extra demands placed on mothers of twins may be no match for their natural hardiness, a study has found, and they may live longer than women who bear children one-at-a-time.
This finding does not mean that there are any health benefits to having twins, rather that healthier women are more likely to deliver babies in pairs.
The source data for the study is both unusual and archaic. Researchers at the University of Utah mined the Utah Population Database, the historical record of the primarily Mormon immigrants to the state from the mid-1800s onwards.
Of the 58,786 women born before 1900 selected from the archive, 4,603 gave birth to twins. They found that mothers of twins went a shorter time between births, had their final child later in life and the time from their first birth to last - their reproductive span - was longer, even when age of marriage was taken into account.
Perhaps the most surprising finding was the correlation with longer life. For women born before 1870, the annual risk of dying after the age of 50 was 7.6 per cent less for mothers of twins than for other women. However, this effect was reduced to statistical insignificance for women born between 1870 and 1899.
The researchers suggest that twinning women’s innate robustness – the factor believed to increase their likelihood of having children in pairs – was particularly important in the rugged pioneer times of the Wild West. As Prof Ken Smith, co-author of the study, says: “When you’re a tougher woman, that toughness is more readily apparent when you are tested by adversity.”
The Utah Population Database is one of the world’s most comprehensive computer genealogies and the authors claim that the sample size for mothers of twins is 18 times greater than for any other similar study. Nonetheless, the database had an odd potential source of bias – polygamy.
Mormons started arriving in Utah in the mid-1800s and in the early days “plural marriage”, as it was known in church doctrine, was widely practiced. Only records for non-polygamous women were selected for inclusion in the study.
The study also only looked at women who lived past menopause and did not include those who died earlier, perhaps in childbirth.
Smith noted: “The women who have twins have a somewhat elevated risk of mortality over those [child-bearing] years, but the vast majority […] reach age 50, and we’re able to observe that they have healthier lives.”
Image credit: Marxchivist (Flickr)
Robson, S., & Smith, K. (2011). Twinning in humans: maternal heterogeneity in reproduction and survival Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.0573
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