Compared with other mothers, women who deliver twins live longer, have more children than expected, bear babies at shorter intervals over a longer time, and are older at their last birth, according to a University of Utah (US) study.
"Having twins will not make you stronger or healthier, but stronger, healthier women are more likely to have twins naturally," says Shannen Robson, the study's first author, and a PhD graduate in anthropology at the University of Utah.
Mothers of twins lived longer after menopause. For women born before 1870, their annual risk of dying after age 50 was a statistically significant 7.6% lower than for moms of singletons. For twinning women born between 1870 and 1899, the annual risk of dying after age 50 was 3.3% lower than for moms who didn't have twins, but wasn't statistically significant.
A woman's innate robustness – the factor that made twins more likely – was more important before 1870 during pioneer times. "When you're a tougher woman, that toughness is more readily apparent when you are tested by adversity," Smith says.
Women who delivered twins had more children than expected due to simply the birth of twins. Mothers in the pre-1870 group averaged 8.39 children while women in the 1870-1899 group averaged 5.72 kids. Mothers of twins averaged 1.9 and 2.3 more children, respectively, than average after controlling for various factors.
"By having twins, you are having one more child by definition, but they exceed the definition and thus had more single births as well," even after controlling for twin moms who had more single births to replace twins who died, Robson says.
Mothers of twins went a shorter time between births. For all moms in the study, the average interval between births was 2.62 years for women born before 1870 and 3.24 years for women born 1870-1899. For both groups, moms of twins averaged two weeks less between births – a time that seems short but is nonetheless important. "Shorter birth interval is an indication of the physical health of the mother," Smith said.
The reproductive span – age at last birth minus age at first birth – was longer for mothers of twins, even when controlling for age of marriage. For women born before 1870, moms of twins had average reproductive spans of 18 years and four months versus 18 years for singleton-only moms. For women born 1870-1899, moms of twins averaged reproductive spans of 14 years 11 months versus 14 years for singleton moms. Both results were statistically significant.
Moms of twins also were older at the time of their last birth. The age at last birth averaged 39.7 years for women born before 1870, and 36.2 years for women born during 1870-1899. Moms of twins had their last births 4.8 months later and 14 months later, respectively. - (EurekAlert!, May 2011)