The puzzle of monozygotic twins
Nowadays, people always associate twinning with in vitro fertilization (IVF). They may be right but there is actually more to twinning than IVF. Especially when it comes to monozygotic twins, commonly known as identical twins. Monozygotic twins develop when an embryo splits into two, resulting into two embryos. The reason for the splitting has always been a puzzle. This is the puzzle that researcher Bruno Reversade is trying to solve as he goes around the world investigating so-called twin villages, as reported in Nature News.
In the general population, the rate of having identical twins is 1 in every 250 to 300 births. In some remote villages in the world, the rate is as high 1 in 10 births. One of these villages is Mohammad Pur Umri in northeastern India, where about 10% of its 2000 inhabitants are monozygotic twins. A stranger who happens to inadvertently walk into this farming village would think he is seeing double. In another case, Reversade traveled to Jordan to interview and collect saliva samples (for DNA testing) from a family with 15 pairs of monozygotic twins. Then his next stop is Linha São Pedro, a Brazilian village predominantly inhabited by German immigrants, where again, 10% of births are twins.
Over the years, human biology experts cannot agree about the mechanisms of monozygotic twinning. Dizygotic twinning is easy enough to figure out because the two embryos come from two eggs which are separately fertilized by sperms. The reasons for multiple egg production have been linked to hormones, maternal age, and well, the number of embryos transferred during an IVF cycle. Monozygotic twinning however is more of a mystery.
Some experts say it’s something to do with genetics, e.g. “it runs in the family”. Other says it’s the environment.
There is also the question whether producing twins is an advantage at all. Some experts believe that at least 12% of natural conceptions can result in twin embryos. However, in most cases, one embryo doesn’t survive so that only about 2% those pregnancies successfully produce twins. The disappearing twin is a common phenomenon. However, there could possibly be even more cases where one twin is lost before it is even detected.
Why a lot of twin pregnancies only deliver one baby is theorized to be due to the fact that a woman’s body is designed to carry only one baby to full term. Having twins, therefore, may actually be an anomaly, a reproduction process gone awry. The disappearing twin is probably a mechanism to correct this. When uncorrected, the pregnancy can result in complications such as the high rate of congenital defects in twins, including conjoined (Siamese) twins. And what about those twins who survive and develop normally? Well, those theorists say it indicates “a healthy system that is able to support two embryos through implantation and pregnancy.” Hmmm… at least something that makes me, mother of monozygotic twins, feel better, for a change.
Whatever findings Reversade’s investigation of the twin villages may reveal will definitely interest me. They might even help me in answering one of the most common questions I hear: Which side of the family has twins? Yours or his?