Twin pregnancies are generally broken down into two types. Monozygotic and Dizygotic. Each has its own subcategories. (Di) meaning two; (Mono) meaning one; and the term zygote or zygotic is a fertilized egg. When you put them together you have dizygotic means two fertilized eggs, and monozygotic means one fertilized egg that is split into two. Dizygotic is commonly known as fraternal twins or twins that not genetically the similar. With monozygotic, it would be identical twins that split into two.
You can have twins one of two ways. With Dizygotic, you can have a woman ovulate two eggs at the same time, which is uncommon, but it does happen. That means each ovary ovulates an egg, and they both find their way into the uterus and get fertilized, so you have two pregnancies going at the same time. Basically, they are siblings, but instead of carrying these siblings in two separate pregnancies, they share the same gestation, and usually birthday too. In this case, a woman’s body decided that she will carry two at the same time. The other type of twins, which is less common for every two or three cases, is monozygotic. This is where one egg is fertilized and splits into two at some point along the early stages of cellular division.
Early Conception of Twins
If the egg splits within the first three days, you have two separate placentas and two separate pregnancies. With twins, there are two separate layers called the Chorion and Amnion. These are the two layers of the embryonic membranes. The one that is further inside is the Amnion, and the one that is on the outside is the Chorion. With normal pregnancies, both act as a single layer. But with twins that are completely separate, they will have two of both layers.
If the egg separates after the first three days, but not after the first seven days, then each baby will develop its own Amnion even though they will share a placenta. That means each baby has a cord running to the same placenta on different sides of the amniotic separation, which is that wall between them. That wall is a membrane that prevents the twins from entangling their cords. If the baby separates after that seven-day zone, then they not only do share a Chorion, but also an Amnion. This means that both Chorion and Amnion are shared by the twins.
Monochorionic and Dyamniotic Twins
Monochorionic and diamniotic twins have special considerations and special risks. If it was actually two eggs, and they both get fertilized, then it’s a dizygotic pregnancy. But if it happens to be a monozygotic twin that developed early, it will act like a dichorionic. How do you know what you are dealing with? You got to look early on because early on, it’s very clear on ultrasound if the pregnancy is two fertilized eggs or one egg that split in half.
Even if you have one egg and one sperm that got fertilized that split into two, if they split early enough, they are going to have their own separate pregnancy. If there had been two eggs that get fertilized, it would act the same way. They will be genetically identical.
Types of twins can overwhelming, exciting and pose some extra monitoring when it comes to the pregnancy itself. At Maternal Resources we give you the calm assurance you need to know we are monitoring you and your pregnancy with the latest and most up-to-date care and information and technology available.
Also check out our podcast on Twins in Pregnancy. Multiple Gestation in Pregnancy
