Multiple Pregnancy Scans

A multiple pregnancy scan is an ultrasound examination performed when a woman is pregnant with twins, triplets, quads or even more. 

Multiple pregnancies make up about 1.8% of all pregnancies. They occur more frequently with

increasing age of the mother as well as with the use of IVF and ovulation induction. Whilst the majority of multiple pregnancies have a good outcome, there are increased rates of complications both for the mother and babies with the commonest complication being premature delivery.

What is done during a multiple pregnancy scan? 

A multiple pregnancy requires careful monitoring due to increased potential for complications.

The multiple pregnancy scan can include several components:

• Determining the number of fetuses and the type of placenta: An early ultrasound can confirm the number of fetuses and whether they share a placenta (monochorionic) or an amniotic sac (monoamniotic), or if they each have their own placenta (dichorionic). This information is vital because pregnancies where babies share a monochorionic placenta or an amniotic sac have much higher risks.
• Monitoring growth: Regular scans are used to monitor the growth of each baby and compare it to standard growth charts. This helps to identify any signs of growth restriction early. 
• Checking for Twin-to-Twin Transfusion Syndrome (TTTS): If the babies share a placenta (monochorionic), they are at risk of  developing TTTS, a condition where blood from one twin (the donor) is transferred to the other (the recipient). 
• Assessing amniotic fluid: The amount of amniotic fluid around each baby will be measured, as imbalances can be a sign of complications such as TTTS.
• Checking fetal anatomy: Each baby’s anatomy will be evaluated in detail to check for any developmental abnormalities.
• Assessing the cervix: The length and appearance of the cervix can be assessed to identify any signs of preterm labour, which is more common in multiple pregnancies.

The frequency and timing of these scans can vary based on the type of multiple pregnancy (e.g., twins, triplets, etc.) and whether or not they share a placenta and/or amniotic sac. They can also vary based on the mother’s health and any complications that may arise during pregnancy.

Understanding different types of multiple pregnancies 

Dichorionic

The most common form of spontaneous twin pregnancy occurs when the mother releases two eggs, and both are fertilised. There are Dizygous or non-identical (fraternal) twins. They will each form their own placenta and this will be a Dichorionic twin pregnancy. If something happens to one twin it won’t directly affect the other twin as they have separate circulations.

Monochorionic

The other form of twins occurs when a single egg is fertilised and this early embryo splits into two. These are Monozygous (Identical) twins. In most of these cases the splitting occurs a few days after conception when this pregnancy has formed one placenta. The two babies will share a single placenta in what is known as a monochorionic placenta. All monochorionic twins will have blood vessels on the surface of the placenta that connect the two twins’ circulations. This means that blood can go from one twin to the placenta, and then through to the other twin. 

There are multiple blood vessel connections, which means that in most cases the flow from one twin to the other is balanced by flow back. However, sometimes the connections are unbalanced and this may lead to major complications such as Twin-twin transfusion (TTTS), acute transfusion TAPS and TRAP.

Diamniotic & Monoamniotic

Most of these twins will be within their own amniotic sac (Diamniotic). If the splitting of the twins occurs late they may share the same amniotic sac (Monoamniotic). If the split occurs very late, they may only partly split which is known as conjoined twins.

3D rendered image of cojoined twins

Without trying to confuse things, identical twins may split very early and each forms their own placenta. So some identical twins may have dichorionic placentas. 

Twin Twin Transfusion syndrome (TTTS)

Twin Twin Transfusion Syndrome (TTTS) is the most common major complication of monochorionic pregnancies. Monochorionic twins will have their own share of the placenta and there are usually multiple blood vessels that connect one twins’ circulation to the other twin. These connections lie on the surface of the shared placenta. This allows blood from one twin to circulate to the other, and this sharing of blood is usually balanced. Most monochorionic twins have their own amniotic sac.

However, in about 10% of monochorionic twins, the connections between them are unbalanced. This results in a transfusion of blood from one twin (the donor) to the other twin (the recipient). This typically develops in mid pregnancy (from 16-28w). 

Both twins will respond to this chronic transfusion differently. The donor twin tries to compensate for the loss of blood volume by decreasing its urine output. As most of the amniotic fluid around the baby is urine produced by the baby, this compensation results in a significant decrease in the amount of fluid seen around this twin. The amnion is seen to wrap around this twin and in the more serious cases there is no fluid seen around the donor twin which now appears to be stuck.

The twin that receives the blood transfusion is known as the recipient twin. It responds to the increased blood volume by significantly increasing its urine output, resulting in excessive amounts of amniotic fluid forming, known as polyhydramnios. The mother may notice a very rapid increase in her abdominal size over the course of a few weeks. If left untreated, the polyhydramnios can increase so much that the mothers uterus can expand to well beyond full term size. This can result in delivery of the twins from either preterm labour, premature rupture of the membranes, or loss of either one or both twins in utero. As TTTS usually develops midpregnancy, very few, if any, of these twins would survive if this condition is untreated.

Other effects of the transfusion process for the recipient twin is that this twin’s heart can enlarge trying to cope with the extra blood volume, and ultimately its ability to pump may fail, resulting in heart failure which can lead to fetal hydrops. 

Due to the TTTS process and the consequences of the transfusion for both twins, its possible  that one or both of the twins may be lost without warning. In this situation, if one twin survives it may develop long term major injury from the TTTS process.

Upon presentation, TTTS can be staged for severity. This guides treatment options. Some twins may present with very early Stage 1 TTTS and they may be observed as this may resolve spontaneously. Any progression or more severe forms of TTTS (stage 2 to 4) usually will be treated by fetal surgery with laser ablation which has a good success rate.

Read more about our co-owner, Dr Rob Cincotta, and his experience treating TTTS in Australia.

Frequency of scans in Twins

In uncomplicated Dichorionic twins we normally recommend regular growth scans about every 4 weeks from 18 weeks to assess growth. It is not uncommon for twins to differ in size and sometimes one may be significantly smaller than the other, affecting timing of delivery. 

In Monochorionic twins we normally would recommend scans about every 2-3 weeks from 16 weeks looking for evidence of Twin Twin transfusion syndrome and other potential complications which can occur due to the monochorionic placenta.