You may be pregnant with twins but you sure don’t know it, at least not yet. You see, Week Two is usually the point that conception occurs. It may seem counter-intuitive or a little confusing to get a two-week head start (shouldn’t conception be Week Zero??) but most health-care professional calculate due date based on the date of your last period since the day you conceived is nearly impossible to nail down. The optimal time for fertilization is during ovulation, usually halfway through your menstrual cycle, approximately two weeks after your last period. So that’s why we begin this series—and you begin your twin pregnancy—with Week Two.
So will your twins be fraternal or identical? Still, too soon to tell. If your ovaries discharged two eggs and both were fertilized by two different sperm, then your twins will be fraternal, or dizygotic (DZ). Two boys, two girls, or boy-girl, the chances are high that your twins will be “siblings born on the same day” as nearly two-thirds of all twins are in fact fraternal. Just like any other siblings, your fraternal twins will share 50 percent of their DNA, at most. Yet sharing a womb, a birthday, and many developmental milestones can create a very strong bond indeed.
It’s a little different story with identical twins or monozygotic (MZ). They occur when a single egg is fertilized by a single sperm. Then something a bit magical happens—within the first week following fertilization (and sometimes even up to 12 days later), this single cell group divides into two identical blastocysts before implanting on the uterine wall. So for a very brief time identical twins are one—they actually begin life as a single entity! Isn’t that cool?
But that’s only half the story with identical twins. Depending on when the cells divide will predict the type of identical twins they will become, the sooner the division happens, the more autonomous the embryos will be. And generally speaking, the more independent the embryos, the fewer complications further along in your twin pregnancy. For instance, if the cells split within the first three days of fertilization, it will be a diamniotic-dichorionic pregnancy, with each baby having his own chorion, placenta, and amniotic sac. If, however, the blastocyst splits between Days 3 and 9, each baby will have his own sac (diamniotic) but share one placenta (monochorionic).
Are you still with me?
But if the cells split between Days 9 through 12, we get into chopper waters as the babies will be monochorionic and monoamniotic with just one sac and one placenta, putting them at risk for twin-to-twin-transfusion syndrome. (Now don’t panic on me just yet as fewer than 15 percent of mono-mono or diamniotic-monochorionic pregnancies result in TTTS.) And finally (yes, there’s more), if the mass splits after Day 12, conjoined twins is the result.
No one knows why the cells split when they do. It’s a pure mystery of life.
And speaking of twin mysteries. How about a third type of twin zygosity? Namely “semi identical,” or half-identical twins. Long been thought of as pure speculation but recently confirmed by scientists, semi identical twins occur when one egg is fertilized by two sperm. The resulting embryo is called a triploid because of its three sets of chromosomes. Scientist believe this extremely rare phenomenon occurs if the egg divides and then each section is fertilized by a separate sperm, or, a single egg fused with two sperm and then each sperm shed either an X or Y chromosome.
Whew! That was a lot of ground to cover but now you’re up to speed on the many types of twins. And you just thought it was as simple as fraternal and identical, didn’t you?