How do twins form?

How and why do twins form?

The phenomenon of twins is truly fascinating not only for its biological intricacies but also for its rarity. Many questions arise such as how twins are formed at conception, the likelihood of different types of twins occurring, the associated risks during pregnancy and how twins develop both in the womb and after birth. Interesting discussions in recent years have examined the factors that may increase the likelihood of having twins and the complications that can arise with twins, particularly during the first three months of pregnancy.

They can be either identical (monozygotic) or fraternal (dizygotic), and their formation is a result of various biological processes. However, there are also different types of identical and non identical twins, depending on for example, whether they are sharing a sac or a placenta.

How are identical twins formed?

Identical twins are also known as monozygotic twins because they are formed from a single fertilized egg (zygote). The fertilized egg splits into two in the very early stages, within the first few days of conception. The earlier the egg splits then the more likely it is that the twins will have their own amniotic sac and placenta (dichorionic twins). If the egg splits a little later, maybe some time around day 4-8 after conception when the blastocyst is more developed, then the babies will be in two separate sacs but share a placenta. These are called monochorionic diamniotic twins (MCDA). These are the most common type of identical twins; around two thirds of all identical twins are MCDA

If the blastocyst splits 9-12 days after conception the babies are more likely to be sharing both a sac and a placenta (monochorionic, monoamniotic). These are more unusual; only around 1-2% of twins separate this late in development. These twins are more at risk of complications during pregnancy and are the rarest type of twins.

Why does the egg split to form twins? How common are twins?

The exact cause of this split is still not clear, but it is believed to be a random event. Identical twins only amount to 3-4 out of every 1,000 pregnancies, and fraternal twins are twice as common at around 8 per 1,000 pregnancies.

How are non identical twins formed?

Fraternal or non identical twins, also known as dizygotic, occur when two separate eggs are released and fertilized by two separate sperm cells. These twins are no more genetically similar than any other siblings born at different times.

Fraternal twins are more common than identical twins. The probability of having fraternal twins varies in different parts of the world and is affected by genetics, ethnicity, and the age of the mother. Older women are more likely to release more than one egg at a time. Studies have shown that as women over 35 have a rise in follicle stimulating hormone, a phenomenon which it is believed is an evolutionary response to combat lower fertility levels and higher risk of abnormalities in embryos.

Do twins run in families?

Identical twins do not run in families; you are just as likely to have identical twins as anyone else. Fraternal twins are influenced by genetics which may mean women in your family or your culture are more likely to release more than one egg at a time. Fraternal twins are more common in Africa than anywhere else in the world

What are the risks of twin pregnancies?

The risk of complications in twin pregnancies varies depending on what type of twins a mother is carrying, as well as other factors such as the mother’s age and overall health. Below are the main risks:

1. Premature Birth

Carrying twins takes a huge toll on a woman’s body. More than half of all twins are born prematurely when the mother goes into labour early, which can lead to neonatal health problems such as respiratory issues, jaundice, feeding problems and developmental delays. However, with appropriate medical care, many premature twins thrive and grow into healthy children.

2. Maternal Complications

Mothers are more at risk of developing high blood pressure in twin pregnancies and will be monitored closely to ensure they are not at risk of pre-eclampsia. Gestational diabetes is also more common than in singleton pregnancies, as is anaemia. Morning sickness is often worse in early pregnancy, as is hyperemesis, and in later months gastro intestinal issues such as constipation are more common.

3. Identical Twin Complications

Identical twins are more at risk of complications than fraternal twins, especially monochorionic monoamniotic twins who are in the same sac and also sharing a placenta. Sharing a sac means there is a risk of potential cord compression or entanglement so the twins will be monitored closely via ultrasound. Dichorionic twins in separate sacs are lower risk.

Twin to twin transfusion syndrome is a complication which can affect monochorionic twins who are sharing a placenta. In this condition, the blood is not shared equally between the twins and this can lead to serious issues with the developing babies, one of whom may be under strain due to receiving two much blood, and have more amniotic fluid around it, and the other will be much smaller due to receiving less blood, and may have too little amniotic fluid around it. If you are diagnosed with twin to twin transfusion syndrome your babies will be monitored very closely with regular ultrasound scans. You may also be offered laser ablation treatment on the blood vessels of the placenta which connect the babies in order to encourage a balanced blood flow.

4. Risk of Miscarriage

Unfortunately miscarriage within the first three months of pregnancy is very common. It is believed that up to one in four pregnancies ends very early and nothing can prevent this, even in singleton pregnancies. The risk of losing a baby within the first two trimesters of a twin pregnancy is slightly higher.

What is Vanishing Twin Syndrome?

If one twin dies in the first three months of a pregnancy, the other twin is often not affected. Sometimes twins can be seen at an early scan in the first few weeks, but at dating scan only one twin can be found at 12 weeks. Sometimes the mother may have experienced no bleeding and has no reason to suspect that an early miscarriage has occurred. In this scenario it is believed that the fetal tissue has been reabsorbed by the mother and the remaining embryo. This reabsorption process is harmless. It is difficult to say how common vanishing twin syndrome is because the majority of women do not have their first ultrasound scan until they are around 12 weeks and many twins may have failed to develop earlier than this.

Twins are an amazing and complex phenomenon, shaped by genetic, environmental, and chance factors. Understanding how different types of twins are formed in the womb, the probabilities of them occurring, and the associated risks and complications is vital for new parents and the clinicians caring for them. Much of it we still don’t understand, yet researchers continue to explore the intricacies of fascinating mystery of aspect of human reproduction. Rest assured that if you find you are expecting twins, you will be monitored very closely throughout your pregnancy.

For more details on twin pregnancies, risks and complications, see the Twins Trust website at https://twinstrust.org/information/pregnancy-and-birth/expecting-more-than-one-baby.html