Introduction

What are triplets?

Triplets are when three babies are born to the same birthing parent at the same time. Triplet pregnancies are much rarer than twin pregnancies.¹

Significance of understanding different types of triplets in pregnancy

There are different types of triplet pregnancies, depending on whether or not the babies share a placenta, or have their own, or whether or not two or more of the triplets share an amniotic sac.

These different types of triplet pregnancies will be discussed later in this article, but why is this important? Well, if two or three of the babies share a placenta they share a blood supply as well. When the blood supply is not equal between the foetuses (the term for a baby when it is in the parent’s womb) this can cause twin-to-twin transfusion syndrome, which can be very serious for the unborn children.

Types of triplets in pregnancy

Identical triplets

Identical triplets occur when an egg is fertilised by a single sperm cell, forming a zygote (a fertilised egg cell). This zygote splits spontaneously, forming three embryos. All three embryos have DNA from the same sperm cell and same egg cell, thus having identical DNA.

• Formation and characteristics: Identical triplets will have identical genes, will look the same, and be of the same sex. They are formed from one egg cell and one sperm cell, meaning that they are monozygotic.
• Occurrence rate: Identical triplets are very rare, occurring 20 to 30 times for every 1 million births.

Fraternal triplets

Fraternal triplets occur when a birthing parent ovulates during their menstrual cycle and releases three eggs, and all three eggs are fertilised by three different sperm, forming three zygotes with different DNA. These types of triplets are known as trizygotic, as there are three non-identical zygotes formed.

• Formation and Characteristics: These triplets can be of a different sex and will not be identical. They share as much DNA as regular siblings do.
• Occurrence rate: A birthing parent may experience infertility issues due to ovulation and their body not producing an egg. A birthing parent will be prescribed fertility drugs which can help combat this, but it can result in a birthing parent releasing multiple eggs when they ovulate, increasing the occurrence rate of fraternal triplets. As a birthing parent gets older there is also a higher chance of them releasing multiple eggs when they ovulate, increasing the occurrence rate of fraternal triplets.

Approximately 80% of triplet pregnancies result in fraternal triplets.

Mixed triplets

When two of the triplets are identical, and the other is fraternal. These types of triplets are known as dizygotic as two eggs are fertilised by two different sperm, forming two fertilised eggs (zygotes).

• Combination of identical and fraternal triplets: This combination arises when a birthing parent ovulates and releases two eggs, both eggs are fertilised, producing two non-identical zygotes, one of the zygotes splits, forming two identical zygotes, which will develop into two identical babies. The zygote that did not split is the fraternal triplet.

Types of triplets sharing placentas and amniotic sacs

The following types of triplet pregnancies, based on whether the foetuses (unborn babies) share a placenta and amniotic sac are listed in the table below:²

Factors influencing the occurrence of triplets

Who has a higher chance of having triplets? There are a number of factors that can result in a birthing parent having multiple babies during a pregnancy which will be discussed.

Maternal age

The age of the birthing parent is a factor affecting the occurrence of triplets.  Birthing parents over the age of 35 are more likely to get pregnant with triplets, as there is a higher chance of them ovulating more than one egg during their menstrual cycle.

Genetics

There are genes that have been associated with having multiple babies. However, more research is needed, and there are likely to be multiple genes influencing this.³

Assisted reproductive technologies

Having triplets may also occur if a birthing parent has had assisted reproductive technologies, such as IVF treatment, to help them get pregnant. This is because, to increase the success rate of IVF, doctors usually implant more than one embryo into the uterus. However, this can result in twin and triplet pregnancies as a result. Assisted reproductive technologies usually result in mixed triplets and fraternal triplets, rather than identical triplets.

Ethnicity

Ethnicity may also influence whether or not a woman has triplets. Black birthing parents have been reported as having a higher rate of multiple births from 2015-2019 compared to other ethnic groups.

Medical considerations and challenges

Pregnancy risks and complications

There are potential complications and pregnancy risks when carrying triplets, which are listed below:

• Anaemia (low iron): the birthing parent may develop a low red blood cell count
• Gestational diabetes: the birthing parent may develop diabetes whilst pregnant due to high blood sugar
• Preterm labour: the birthing parent may go into labour before 37 weeks
• Premature birth: the babies may be born before full term is reached
• Polyhydramnios: when there is too much amniotic fluid
• Pre-eclampsia: when the birthing parent develops high blood pressure prior to being 20 weeks pregnant
• Placental abruption: this affects the unborn babies when the placenta detaches from the uterus, and the foetuses may not receive enough blood flow and nutrients from the birthing parent
• Twin to twin transfusion syndrome: this affects triplets that share a placenta, where one may receive less blood flow than another.

Preterm birth and low birth weight

Being pregnant with triplets can increase your chance of having them preterm, and there is limited guidance on how to prevent preterm birth.⁴ Low birth weight is also common amongst triplets where they weigh less than 2.5kg, which can give rise to a range of health issues, including difficulty breathing.

Neonatal intensive care

Babies who are born preterm or with a low birth weight may need to go to neonatal intensive care, where the babies receive specialist care due to possible health problems they may have from being born early or with a low birth weight. Babies in this unit may need an incubator or a breathing machine if they have difficulty breathing.

Emotional impact on families

Emotionally it can be very distressing if your babies are born with any form of health condition, especially when this requires neonatal intensive care. However, you may still be able to help with feeding and changing the babies’ nappies.

Diagnosis and prenatal care

Early detection of triplets

Triplets may be able to be detected early if the birthing parent is putting on a lot more weight than expected, or there are higher levels of human chorionic gonadotropin (hCG) in their blood (which is produced by the embryo). If there are three embryos then we would naturally expect this to be higher. In addition, hearing more than one heartbeat will also confirm that you are pregnant with triplets, as well as a prenatal ultrasound which will show three foetuses in your uterus.

Specialised prenatal care

Due to the higher risk of complications when you are carrying triplets, more frequent checkups are required than if you were pregnant with one baby. During the first and second trimester, checkups may be every 2 weeks. However, during the third trimester, as the due date becomes closer, weekly checkups with a midwife will be necessary.

Monitoring the health of the birthing parent and the babies

Monitoring the foetuses health can occur via ultrasound or via amniocentesis, where a needle is inserted into the amniotic sac and a portion of amniotic fluid is extracted. The amniotic fluid will contain the DNA of the foetuses and can be used to test for any genetic conditions that the triplets may have. An alternative to amniocentesis is a chorionic villus sampling where cells from the birthing parent's placenta are removed and tested for genetic conditions. For the birthing parent, blood tests may be conducted to see if she has gestational diabetes or low iron, which may lead to anaemia. The birthing parents blood pressure will also be monitored throughout the pregnancy for high blood pressure.

Summary

• Triplets can be identical, fraternal or mixed. This depends on how many eggs are fertilised
• Prenatal care is very important to check that the foetuses are developing properly due to the higher risk of complications, especially if the triplets are sharing a placenta
• The types of twins vary depending on whether or not they share an amniotic sac and/or placenta
• Prenatal care is also important for the mother as she is at a risk of developing gestational diabetes, high blood pressure, and anaemia
• Preterm births and low birth weights are more common in triplet pregnancies, which may result in the babies being in neonatal care