With twin pregnancies often presenting as emergencies to the obstetric team, this week we dive into the embryology and how twins develop.
There are two mechanisms which can produce twins:
- Monozygotic – where one sperm fertilises one egg but this subsequently splits
- Dizygotic – fertilisation of two separate eggs occur
The dizygotic twin will not be identical and occurs in around 70% of twin pregnancies, whereas monozygotic twins will be identical.
In monozygotic division can occur at different points in early pregnancy. Depending on the day post conception that the split occurs determines the fetal environment. If the division occurs early on in, before day 3, then each foetus has their own placenta and amniotic sac. Whereas if the division happens after day 9 then the two babies share a placenta and an amniotic sac. If the division occurs between these, then the babies share a placenta but have separate amniotic sacs. This is due to the times at which the cells differentiate to form the placenta. On day 4 trophoblasts start appearing to form a sphere inside which is the inner cell mass. The trophoblasts are the cells that develop to form the placenta and outer layer of membrane encapsulating the foetus. The inner cell mass will form the foetus and the amniotic sac, therefore splitting at this stage leads to one placenta but two amniotic sacs. On day 9 the inner cells differentiate to form the yolk sac and amniotic sac, so splitting at this point means the babies share both a placenta and an amniotic sac.
A complication of having twins is something called twin to twin transfusion. This only occurs in twins that share a placenta. The blood vessels connect within the placenta and divert blood from one foetus to the other. Over time this can result in the recipient foetus receiving a much greater blood supply than the donor foetus. Leading to a greater blood volume in the recipient therefore strain on the cardiovascular system and a greater volume of amniotic fluid. Sometimes delivery is the only treatment.