Week 10 of my twin pregnancy was a big milestone for that’s when my husband and I decided to announce to our families and friends that we were expecting more than one. By ten weeks, my babies had passed one of the most critical milestones of development and now were no longer embryos but fetuses. With the risk of miscarriage slowly diminishing with each passing day, we felt as though we could relax just a bit and share the good news with our loved ones. Besides, by Week 10 my baby bump was already beginning to show!
With the end of your first trimester just a few weeks away, the real fun begins. You may have full-blown morning sickness but that’s because your twins are busy growing at a rapid pace building tissues and organs. All the major ones are in place—kidneys, brain, liver—and are starting to function. Eyelids are forming and so are red blood cells. Plenty more prenatal tests are in your future but I won’t bore you with those details since you can find that information anywhere on the Internet including right here. Instead, I’ll focus this post on something more fun—intrauterine behavior. Namely how your twins are interacting in your womb.
Do Twins Interact in the Womb?
Although you won’t feel your babies kicking for several more weeks, it doesn’t mean that they are not moving around. In fact, by Week 11 or 12 there’s plenty of movement but since they’re still too tiny and buried too deep within your womb you won’t be aware of it. Unlike singletons, twins share their mother’s uterus, leading to all sorts of myths and legends about what really goes on in there (even the Bible has a few stories of dueling twins in utero). But with the advent of ultrasound in the late 1970s coupled with the work of Dr. Alessandra Piontelli, a twin researcher in the Department of Maternal/Fetal Medicine and Perinatology, University of Milan, the mystery has been solved. Twins do physically interact with each other in the womb!
During the latter half of the first trimester, twin fetuses are still relatively small making it possible to view and study both by ultrasound at the same time. (Sadly, by Week 15 the fetuses are just too big to be viewed together.) In her research, Piontelli has observed that monochorionic twin pregnancies (identical twins who share one placenta) begin intratwin stimulation sooner than dichorionic twins (fraternal twins with separate placentas). By Week 11, monochorionic twins have some physical contact due to their close proximity and their relatively thin membrane separating their amniotic sacs, albeit minor. Dichorionic twins, on the other hand, don’t begin their physical communication until Week 12 or 13. Piontelli believes that the delay can be attributed to fraternal twins implanting in different areas of the uterus and the thicker membranes dividing their amniotic sacs.
As your twins continue to grow so do their interactions with each other. Around Week 14, twins may begin to actively and deliberately reach out to their cotwins. They may touch each other’s heads and arms. One researcher even observed one twin caressing the back of a cotwin at Week 18. By Week 20, Piontelli says that nearly one-third of all twin movement is reactive, meaning “Twin A” may move solely because “Twin B” touched him or even kicked him.
Although, I’m sure, it was totally by accident!
Nonetheless, it’s fascinating to think that your twins’ bond is already evolving even before they’re born.