Learn about the scan that closely examines twins, triplets and higher order pregnancies to ensure holistic health.

Summary

  • A multiple pregnancy is one where the mother is carrying more than one child, like twins, triplets or more.
  • Twins can be non-identical or identical
  • The risk of certain complications is higher with multiples. Starting in the second trimester, the patient may have ultrasound exams every 2–4 weeks. For e.g. Multiples are more likely to have growth problems than single babies. Multiples are called discordant if one fetus is much smaller than the others.
  • Certain scans such as a chorionicity scan are done additionally in case of multiple pregnancies
  • Risk of disorders such as Down syndrome is higher in case of a multiple pregnancy.
  • It is not possible to be as accurate in determining this risk in multiple pregnancies as it is when there is just one baby. As a result of this patients have a greater likelihood of being offered invasive testing.
  • Identical twins sometimes experience what is called Twin to Twin transfusion syndrome where one twin gets more blood than the other. In such cases laser treatment or amniodrainage is offered.

What is multiple pregnancy?

A twin, triplet, or higher-order pregnancy (four or more babies) is called multiple pregnancy. If more than one egg is released during the menstrual cycle and each is fertilized by a sperm, more than one embryo may implant and grow in the uterus. This type of pregnancy results in fraternal twins (non-identical). When a single fertilized egg splits, it results in multiple identical embryos. This type of pregnancy results in identical twins (or more). Identical twins are less common than fraternal twins.

What are some causes of multiple pregnancy?

The use of fertility drugs to induce ovulation often causes more than one egg to be released from the ovaries and can result in twins, triplets, or more. In vitro fertilization can lead to a multiple pregnancy if more than one embryo is transferred to the uterus. Identical multiples also may result if the fertilized egg splits after transfer. Women older than 35 years are more likely to release two or more eggs during a single menstrual cycle than younger women. Therefore, they are more likely than younger women to become pregnant with multiples. Other reasons are if the patient has a family history of twins on her mother’s side or if she has had a multiple pregnancy before.

Difference between identical and non-identical twins

The babies can be identical or non-identical.

  • Identical babies are called monozygotic. Monozygotic siblings come from one egg. In this case the egg has been fertilised by one sperm and then split into two or more embryos. These babies will all be the same sex. They will all have identical genes.
  • Non-identical twins are called dizygotic . Triplets or higher multiples are polyzygotic. In this case each egg is fertilised by a separate sperm. The babies are not necessarily the same gender and they have different genes.

As the early ball of cells implants itself into the womb, the lining of the womb starts growing another type of tissue called the placenta. The growing embryo is attached to the placenta through a tube called the umbilical cord. The placenta supplies all the nutrients and blood to the growing fetus. The fetus grows within a bag of fluid called the amniotic sac. The inner lining of the amniotic sac is called the amnion. The outer lining is called the chorion, and connects to the placenta.

Dizygotic fetuseshave their own placenta, amnion and chorion. Each fetus develops separately in its own sac with its own blood supply. Each placenta may be separate or fused together.

In identical (monozygotic) pregnancies, however, this situation is more complicated. It depends on exactly when the zygote divided into two or more zygotes. There may be one placenta or more than one. If there is one shared placenta, this is called a monochorionic pregnancy. There can be various different types of monozygotic pregnancies - for example:

  • Twins can be dichorionicdiamniotic. The twins have separate amnions and chorions. This is the type of twin pregnancy with least risk.
  • Twins can be monochorionicdiamniotic. The twins share the same chorion and placenta. They grow in separate amniotic sacs.
  • Twins can be monochorionicmonoamniotic. The twins share the same amnion, chorion and placenta.
  • Siamese twins (conjoined twins). The fetuses themselves are joined together physically. It is very rare.
  • Triplets can be:
    • Trichorionic. Each baby has its own placenta and chorion.
    • Dichorionic. Two of the babies share a placenta and chorion and the other is separate.
    • Monochorionic. All three babies share the same placenta and chorion.

These different types of multiple pregnancy have different risks and problems attached to them. Antenatal care is therefore different depending on which type of multiple pregnancythe patient has. Generally, the more babies there are, the higher the risk.

In twin pregnancies, one twin can receive a reduced blood supply and have a slower growth rate (twin-twin transfusion). Sometimes, there is a small risk of miscarrying one fetus.

Type of Scans done in multiple pregnancies

The extra scans that can be performed for multiple pregnancies are:

  • A chorionicity scan at the same time as the dating scan and combined screening test. This is to check whether the babies share a placenta, or have one each.
  • Growth scans in the third trimester will check that the babies are growing normally. Babies sharing a placenta will be scanned every two weeks from 16 weeks. Those with their own placenta will be scanned every four weeks from 20 weeks.

Scanning twins is more of a challenge than scanning one baby, because one twin is often behind the other.

If one baby's head is low in the pelvis, it would be difficult  to get any measurements. The scans will take more time than they would for a single pregnancy.

It may not be easy to check the babies' growth, and scans will not tell exactly how big the babies are. But by measuring them each time during a scan it's possible to get an idea of whether they’re growing normally. 

A variety of measurements have to be taken into account to get an accurate picture of the babies. Where possible, measurements of the babies' heads, abdomen and thigh bones will be taken. 

Difference in the size of twins, whether identical or not, is expected and normal. It's only when one twin is more than 25 per cent bigger than the other, or growing much faster, that there may be a problem. 

Despite an increased number of scans, it has to be noted that there is no increased risk to the mother or the babies.

  • Differences in fetal weight should be monitored using two or more parameters at each ultrasound scan from 20 weeks.
  • Mothers with uncomplicated dichorionic twin pregnancies should be scanned every 4 weeks from week 20.
  • Mothers with uncomplicated monochorionicdiamniotic twin pregnancies should be scanned every 2 weeks from week 16.
  • Monochorionic twins should be scanned fortnightly to detect TTTS from 16 to 24 weeks of gestation.
  • Women with uncomplicated trichorionictriamniotic triplet pregnancies should be scanned every 4 weeks from week 20.
  • Women with uncomplicated monochorionictriamniotic and dichorionictriamniotic triplet pregnancies should be scanned every 2 weeks from week 16.
  • If there is a 25% or greater difference in size between twins or triplets, this is a clinically important indicator of growth restriction.

Prenatal diagnosis

It should be noted that in a multiple pregnancy, there is a greater likelihood of Down syndrome, hence different options for screening and a higher false-positive rate of screening tests.

In case of monozygotic twins, the risk is the same for each twin. However if the babies are not monozygotic, the risk of Down's syndrome will be different for each. It is not possible to be as accurate in determining this risk in multiple pregnancies as it is when there is just one baby. As a result of this patients have a greater likelihood of being offered invasive testing and of complications occurring from this testing.

Screening tests for genetic disorders that use a sample of the mother’s blood are not as sensitive in multiple pregnancy. It is possible to have a positive screening test result when no problem is present in either baby.

Invasive tests for birth defects include chorionic villus sampling and amniocentesis. These tests are harder to perform in a multiple pregnancy because each fetus must be tested. There also is a small risk of loss of one or all of the fetuses. Screening should be performed between approximately 11 weeks 0 days and 13 weeks 6 days during which:

  • The Fetal positions should be noted.
  • The risk per pregnancy in monochorionic pregnancies and for each baby in dichorionic and trichorionic pregnancies should be calculated.
  • Triplet pregnancies should use nuchal translucency and maternal age. It is not advisable to use serum screening.

If one fetus is detected as abnormal, selective termination (if desired) must be accurately targeted. Selective termination in monochorionic pregnancies could pose risksto the co-twin.

Is the risk of complications higher with multiple pregnancy?

The risk of certain complications is higher with multiples such as:

  • There is an increased risk of stillbirth.
  • There is an increased risk of preterm birth.
  • Twin-twin transfusion syndrome (TTTS) occurs commonly in monochorionic twins. But it also occurs in monochorionic and dichorionic triplets.
  • There is an increased risk of umbilical cord entanglement, mainly in monochorionicmonoamniotic twin pregnancies.
  • There is an increased risk of IUGR.
  • There is an increased risk of congenital abnormalities.

What is Twin to Twin Transfusion Syndrome?

Twin-to-twin transfusion syndrome (TTTS) can affect identical twins who share a placenta (monochorionic twins). 

TTTS happens when there is an imbalance in the placental blood vessels that connect both twins. If the blood doesn't flow evenly between the twins, one gets more blood, called the recipient twin, while the other gets less blood (the donor twin). 

The uneven blood flow results in the recipient twin growing bigger, due to the extra nutrients and fluid than the donor twin. 

The extra fluid the recipient twin takes in can put a strain on the heart. The body will try to get rid of the fluid by producing more urine. As a result, the recipient will have too much surrounding amniotic fluid, while the donor will have little or none. 

The recipient twin and the extra fluid can press the donor twin against the wall of the womb (uterus). This may make the mother uncomfortable and can even result in contractions.

The vast majority of twins who share a placenta grow normally. A majority of the monochorionic twins don't develop TTTS. TTTS can be very serious if it is not treated. However, treatment is successful in a significant per cent of cases.

The scans for TTTS take place every two weeks from 16 weeks to 24 weeks. If there are signs that TTTS may be developing, the patient should have weekly scans and undergo further treatment.

The fetal medicine specialist can offer multiple options. One is to drain amniotic fluid from the fuller sac (amniodrainage). This may help to correct the imbalance and reduce the pressure inside the womb in mild or moderate TTTS. 

In severe cases of TTTS, before 26 weeks, there is another option. The specialist may be able to use a laser to close the vessels in the placenta. A laser beam is used to destroy the abnormal vascular connections connecting the two twins. It is performed by a Fetal Medicine specialist under anaesthesia. The procedure takes about 30 to 60 minutes. A tiny telescope is inserted into the amniotic cavity through a minute incision on the mother’s abdomen. The abnormal vessels viewed are then closed using heat.

This will stop the blood imbalance supply between the twins. The specialist may also drain some of the excess fluid at the same time. 

While laser treatment is a permanent treatment for TTTS, amniodrainage has to be repeated. However, laser treatment is more complex.

There is a risk of the patient’s water breaking, the placenta coming away from the womb wall (placental abruption), or miscarriage.

If the patient undergoes treatment for TTTS, then she’ll have to be scanned regularly to check that it's resolved the problem and that the babies are growing well. This may be at least weekly until birth. 

Babies affected by TTTS are more likely to be born prematurely.

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