What are monozygotic twins?
Monozygotic twins are identical twins and form when a single fertilized egg splits. Depending on the timing of the spilt, the twins will have different numbers of placentas (the organ that nourishes the fetus inside the womb) and amniotic sacs. Monozygotic twins may have two separate placentas and two separate sacs, called dichorionic/diamniotic. This is the same number of placentas and sacs that occurs with fraternal (non-identical twins). Dichorionic/diamniotic twins are at increased risk for complications compared to a single fetus pregnancies, but are the lowest risk type of twins.
Monozygotic twins may also share one placenta but have two separate sacs. These are called monochorionic/diamniotic twins. Sixty to 65 percent of identical twins are monochorionic/diamniotic. Monozygotic twins sharing one placenta and one sac are called monochorionic/monoamniotic twins. Finally, monozygotic twins that are monochorionic/monoamniotic can be conjoined.
The number of placentas and sacs are best determined by an ultrasound before week 14 of the pregnancy. This is when the placenta and amniotic sac (bag of waters) are most visible to your doctor. All twin gestations require frequent monitoring by ultrasound and other fetal testing as the pregnancy progresses. Depending on the number of placentas and amniotic sacs, and the presence or absence of other conditions, our doctors will diagram an individualized plan for you and the care of your babies. As a minimum, dichorionic/diamniotic twins should have ultrasounds every 4 weeks.
How do anomalies in monozygotic twins affect my pregnancy?
Some issues the twins might face include:
- Selective fetal growth restriction
- Weight inequality
- Low birth weight
- Different amniotic fluid levels
- Twin-Twin transfusion syndrome
- Umbilical cord entanglement or compression
- Twin-reversed arterial perfusion sequence
We recommend that all mothers — whether they are carrying monozygotic twins with anomalies or any twins — get plenty of rest and focus on eating well. Your doctor will talk to you about bed rest, certain medications to stop contractions or labor and other ways to prevent an early delivery. You will have more prenatal visits with your doctor than moms not facing complications.
How do you treat anomalies in monozygotic twins?
As with all twins, be prepared for an early labor and delivery. The rate of very early delivery (before 32 weeks) is nearly twice as high for monozygotic twins than other twins. Odds also increase for a cesarean section.
Here are a few complications that might arise and how we treat them:
- Cord entanglement. If this is the case, doctors might recommend that your twins be delivered as early as week 24. Your doctor might give you medication to help accelerate the lung development of your twins to help them survive outside of the womb. Your doctor will probably perform a cesarean section delivery.
- Amniotic fluid issues. If excessive fluid is a problem, your doctor will perform a procedure to reduce the amount of fluid that will make you feel more comfortable, help equalize blood flow between twins and reduce the chance for premature labor.
- Twin-Twin transfusion syndrome.
- Twin-reversed arterial perfusion sequence.