A multiple pregnancy is when you are pregnant with more than one baby. You might be expecting twins, triplets or more!

In Australia, about 3 in 100 births are multiple babies. And most of these are twins.

How do multiple pregnancies occur?

Multiple pregnancies occur when more than one embryo implants in your uterus (womb). This can happen:

• if you release more than one egg during your menstrual cycle and each egg is fertilised by a sperm
• your fertilised egg splits into 2, resulting in 2 identical embryos

Multiple pregnancies are more common than they used to be. This is mainly because of the increasing use of in vitro fertilisation (IVF). The fertility medicines used in IVF often cause more than one egg to be released from the ovaries.

Sometimes with IVF, more than one fertilised embryo is transferred to the uterus. This can result in a multiple pregnancy.

Another way a multiple pregnancy can occur is if one of the transferred eggs splits in two. This results in twins.

Women aged 35 years and older are more likely to release more than one egg when they ovulate. This means that you are more likely to have non-identical twins as you get older.

You are also more likely to have a multiple pregnancy if there is a history of non-identical twins on your mother’s side of the family. Identical twins do not run in families.

Diagnosis of multiple pregnancy

Signs that you might be expecting multiple babies include:

• you seem bigger than you should be for your dates
• twins run in your family
• you have had fertility treatment

A multiple pregnancy is confirmed by an ultrasound scan. This is usually in the first trimester (the first 12 weeks of pregnancy).

The ultrasound will confirm the type of multiple pregnancy. This includes:

• the number of placentas
• the number of amniotic sacs

These are all important factors for later in the pregnancy. It's important to identify these factors as early as possible.

If you are carrying multiple babies, you will have to see your doctor or midwife more often than someone who is expecting one baby.

Most multiple pregnancies progress smoothly. However, there's a higher chance of your babies:

• being born prematurely
• having a low birth weight

You may also have complications with your pregnancy, such as:

• gestational diabetes
• pre-eclampsia
• premature labour
• bleeding

Types of multiple pregnancy

The most common type of multiples include the following.

Fraternal twins

Two separate eggs are fertilised and implant in the uterus.

Your babies are:

• siblings
• share the same uterus
• may look similar or different
• may be the same gender (2 girls or 2 boys) or different genders (a boy and a girl)

A pregnancy with fraternal twins has the lowest risk of all multiple pregnancies. This is because each baby has its own placenta and amniotic sac.

You will sometimes hear fraternal twins referred to as 'dizygotic' twins. This refers to 2 zygotes (fertilised eggs).

Identical twins

Identical twins are formed when a single fertilised egg splits in 2.

Each embryo is genetically identical, so the babies share the same DNA. That means that your babies will share many characteristics. But sometimes identical twins can look quite different. This is because their appearance is also influenced by the environment.

Identical twins may:

• share the same placenta and amniotic sac
• share a placenta but have their own amniotic sac
• have their own placenta and amniotic sac

You will sometimes hear identical twins referred to as 'monozygotic'. This refers to one zygote (fertilised egg).

Triplets and 'higher order multiples' (HOMs)

Triplets, quadruplets, quintuplets, sextuplets or more can be a combination both of identical and fraternal multiples.

For example, triplets can be:

• fraternal (trizygotic), forming from 3 individual eggs that are fertilised and implant in your uterus
• identical, when one egg divides into 3 embryos
• a combination of both fraternal and identical babies

Getting support

If you are having multiple babies, you will need a lot of support throughout your pregnancy and after the babies are born.

For support and resources, you can contact the Australian Multiple Birth Association. You can also call Pregnancy, Birth and Baby on 1800 882 436 for advice or support.