Triplet pregnancies can come in a few combinations of shared embryonic sacs, shared placentas, identical, fraternal etc. The types of triplets you are carrying can affect the likelihood of experiencing complications during your pregnancy. Triplet pregnancies in themselves are pretty rare. Recent (2020) figures from the CDC show a triplet or higher order birth rate of 79.6 per 100,000 live births. Higher Order Multiples (HOMs) is a term used to describe triplets, quadruplets, quintuplets and more.
What are the possible combinations or types of triplets and higher order multiple pregnancies?
Triplets can be either fraternal or identical siblings, but the different types of triplet combinations doesn’t stop there. Whether your babies share a placenta or an amniotic sac also dictates the level of risk in both pregnancy and delivery. I will first explain the combinations of triplets when describing their genetic relationship to their siblings and then how they are classified according to the sharing of placentas and amniotic sacs.
Identical Triplets (Monozygotic):
Identical triplets occur when an egg splits after being fertilized and then splits again and so have nothing to do with assisted fertility treatment – they are completely spontaneous. Identical triplets are rare, estimated at one in every 76,000 births. There is no real explanation as to why eggs (zygotes) split. It occurs in the very early stages (up to a week) after fertilization when the zygote is no more than a cluster of a few cells. The earlier the split happens, the more independent the embryos will grow from one another. Later splits may result in embryos sharing placentas, amnions and chorions. It is believed that there is a higher chance of identical twins or more in older women as egg quality declines, although this occurrence can take place in women of any age.
Fraternal Triplets (Trizygotic or Polyzygotic):
The third combination is three fraternal (non-identical) siblings, created by three separate eggs and three sperm. There are several reasons why a woman may release several eggs at a time. Often, a woman may experience infertility due to issues relating to ovulation and will be prescribed fertility drugs to assist with this. A side effect of these drugs is the increased chance of releasing more than one egg at a time. Genetics can play a part on women releasing multiple eggs and it is fraternal twins or multiples that tend to run in the family, not identical ones. As a women gets older, she is also more likely to release multiple eggs at a time.
A common misconception by the public is that multiple births are often a result of placing numerous embryos in the uterus during IVF treatment. In some cases, twins, triplets or even higher numbers of multiples do occur after more than one embryo is transferred, it really only accounts for very small percentage of Higher Order Multiples overall. The implantation of many embryos is commonplace in the United States, however, doctors in Australia are reluctant to implant more than one embryo and will often only consider implanting two when there have already been several unsuccessful attempts with one embryo. A senior fertility specialist and spokesperson for the Fertility Society of Australia, Professor Michael Chapman, stated that of the 57,000 embryo transfers than occur in Australia every year, less than 1000 are triple embryo transfers.
Mixed Triplets (Identical pair plus one)
When a woman released two eggs, which are both fertilized by separate sperm and then one splits, we end up with a set of identical twins plus a third sibling. This is the most common variation of triplets.
Types of triplets sharing placentas and/or amniotic sacs
Multiple pregnancies are also classified in relation to whether the babies share one or more placentas (otherwise known as “chorions”) and whether or not they share an amniotic sac. Of course every pregnancy is unique, but generally speaking, the more independently the babies are growing, the lower risk of pregnancy complications and the more sharing of placentas and sacs, the higher the risk of complications .Monochorionic fetuses share one placenta; dichorionic fetuses occur when two share a single placenta and one has their own. Trichorionic triplets each have a separate placenta.
Monochorionic Triplets share a single placenta.
Dichorionic Triplets share two placentas, so one will have its’ own.
Trichorionic Triplets each have their own placenta.
Monoamniotic Triplets share a single amniotic sac.
Diamniotic Triplets share two amniotic sacs between the three of them, so two will share.
Triamniotic Triplets each have their own amniotic sac.
There are many combinations possible – sometimes two babies may share a placenta, yet each has his/her own amniotic sac. This example would be classed Dichorionic/Triamniotic Triplets.
Hopefully this simple breakdown of the three types of triplets has cleared things up a little, for those that are just beginning their search for information. If you, or someone you know is expecting a multiple birth, please get in touch via our “Contact Us” page to be connected to other families who will be able to offer advice and insight in to what lies ahead.