Seeing Double: How Do Twins Form?

By Eva Martin, MD of Elm Tree Medical, Inc.

There are few bigger surprises than learning that you are pregnant with twins. We can all picture the shocked mother and father looking at an ultrasound of two growing babies with wide eyes. The incidence of twins is on the rise, mostly due to the increased utilization of ART. With this increase, researchers have taken another look at the mechanisms that lead to twins, both "typical" and "atypical." This week, we cover the traditional and some new models for how twins form.

Typical Twinning: Traditional Models

Every medical student learns about monozygotic and dizygotic twins, commonly known as "identical" and "fraternal" twins. We have long believed that monozygotic twins (identical twins) result when a single egg and sperm divide to form 2 embryos. When exactly this single egg-sperm combo splits is believed to determine what the two fetuses share during pregnancy (one or two placentas, one or two amniotic sacs).  In ART pregnancies, there is a 2 to 5 fold increase in monozygotic twins, perhaps due to handling of the embryo prior to implantation. On the other hand, dizygotic twins result from two separate eggs being fertilized by two separate sperm. These infants have completely different genes and different fertilization events.

Atypical Twinning: New Models

Four years ago, in 2013, Dr. Herranz challenged these traditional models of twinning. How do we know that one egg-sperm combo splits to form twins? We've never seen it. What if the split occurs very early in embryo formation and then the placenta or amniotic sac fuse later in pregnancy? That same year, Dr. Denker published his opposition to Dr. Herranz's new theories... and the old traditional splitting models, too!

The first atypical twinning theory is "Chimeric Twins." A chimera is a single organism that has 2 genetically distinct populations of cells. The incompletely-defined theory is that two eggs are separately fertilized by two sperm, and later, the chorion (placental cells) fuse to create twins that share a placenta but not an amniotic sac. A second hypothesis holds that one egg is fertilized by two sperm, splits, and then the chorion re-fuses later.

The second atypical twinning theory is "Polar Body Twinning." When cell division occurs to create an egg, small cellular byproducts also form: the polar bodies. The polar bodies usually burst and disappear within a day of forming. However, the Polar Body Twinning theory posits that it is possible for a sperm to fertilize the polar body. In this case, one sperm fertilizes the egg and another sperm fertilizes the polar body. Two rounds of cell division result in polar bodies. If the polar body from the first round of cell division is fertilized, the resulting fetus will have an incorrect number of chromosomes. The polar body from the second round will have the correct number of chromosomes.

The third and final atypical twinning theory for today is "Superfetation" and "Superfecundation." Superfetation refers to a second pregnancy implanting after another pregnancy has already started. The theory is that after a pregnancy has started, the woman ovulates again and a second sperm fertilizes this new egg, resulting in a second pregnancy implanting. It's unknown if this is possible, but, if it did occur, the twin from the second pregnancy would be significantly smaller/younger than the twin from the first pregnancy. Superfecundation refers to two separate eggs from the some ovulatory cycle being fertilized on two separate occasions. Theoretically, superfecundation could explain twins with different fathers.

We want to hear from you! Do you believe any of these new theories of twinning? Have you heard of other theories?