United States moms set a record in 2014, with 33.9 out of every 1,000 births turning out to be twins. Although insignificant statistically, the increase from the 2013 rate of 33.7 per 1,000 births still qualified as “a new high for the nation,” a recent report by the National Center for Health said.

In a racial perspective, 78,788 of all the twins were born to white women, 23,546 were born to black women and 22,051 to Latinas, according to the report.

A set of twins in US

The total number of twin births in the United States in 2014 stood at 135,336. Ranking fifth on the historical record list, it is 2.3 percent higher than in the previous year but 2.6 percent lower than the all-time high of 138,961 twin births, which occurred in 2007.

As for triplets or higher-order multiples, the birth rate in 2014 has been recorded as 113.5 per 1,000 births, down more than 40 percent from the 1998 peak (193.5) and the lowest in two decades.

The reason for an increased possibility of having twins is two-fold: the rise in the average age of first-time moms in the United States on the one hand, and a more conservative attitude toward in vitro fertilisation among the American moms on the other hand.

The average age of first-time moms in the United States reached a record high of 26.4 years old in 2014, with birth rate for women in their teens and early twenties falling and that of women aged between 25 and 40 rising. Meanwhile, doctors have been encouraging women to transfer fewer embryos to their wombs even though in vitro fertilisation is becoming more successful.

Scientists have discovered that when a woman gets older and thus her ovaries run low on eggs, the body tends to release higher levels of what is known as follicle-stimulating hormone, which in turn contributes to the higher likelihood of the release of multiple eggs.

However, the aging factor is only explicable for fraternal twins, triplets or higher-order multiples, which are the results of two or more eggs being in good quality. Whereas for conceiving identical twins, which occur when a single fertilized egg splits into two, the mom’s age is not a relevant factor.