Multiple Pregnancies

What is a Multiple Pregnancy?

A multiple pregnancy, also known as a multiple gestation, occurs when a woman is carrying more than one fetus in her womb simultaneously. The most common types of multiple pregnancies are twins (two fetuses) and triplets (three fetuses), although more complex cases can involve higher numbers.

Multiple pregnancies can result from the release and fertilization of multiple eggs (dizygotic or fraternal) or the splitting of a single fertilized egg into multiple embryos (monozygotic or identical).

While multiple pregnancies can bring great joy and anticipation, they also pose unique challenges and considerations due to the increased demands on the mother's body and the potential for higher risks and complications. Adequate prenatal care, monitoring, and support are essential for ensuring a healthy outcome for both the mother and the developing babies.

What are the types of Multiple Pregnancies?

Twins (2 babies in the uterus) are the most common type of multiple pregnancy, accounting for >90% of multiple births.

First we will look at two genetic types of twins (medically termed zygosity):

Dizygotic twin pregnancy

Where two eggs are fertilised in the same cycle (65% of all twins). These are sometimes called fraternal twins as they have the same genetic makeup as do brothers and/or sisters.

Dizygotic twins tend to occur more frequently with increasing maternal age, couples with a family history of twins, particular racial types and from assisted reproductive technology (eg IVF, ovulation induction).

Monozygotic twin pregnancy

Where a single egg is fertilised but later divides to form two fetuses (35% of all twins). These are sometimes called identical twins as they have virtually the same genetic makeup as each other.

Monozygotic twins tend to occur with equal frequency across maternal age and race. They may also be associated with ovulation induction and blastocyst transfer techniques.

What is Chorionicity?

During pregnancy we often cannot determine the zygosity of twins (unless one is a boy and the other a girl). You will hear the term chorionicity mentioned rather than zygosity. Chorionicity refers to the type of placentation in a twin pregnancy and is used to stratify obstetric risk level.

The chorionicity of a twin or higher order multiple pregnancy can be accurately determined at the time of First Trimester Screening or earlier if an ultrasound scan has been performed in the early first trimester. Amnionicity (or ascertainment of the number of sacs) is only accurately assessable after 8 weeks gestation.

There are two types of placentation in twin pregnancies:

Dichorionic placentation (most frequent: 80% of all twins)

Dichorionic twin placenta demonstrating 
the 'Twin Peak' sign

A dichorionic twin placenta means that there are two separate placental discs – they may be widely separated in the uterus or in close proximity depending upon where the two embryos implanted in the uterus. All dichorionic twins have two separate sacs (medically termed amniotic sacs) in which the babies live. Thus this sort of twinning is described as dichorionic and diamniotic (two placentas and two fluid-filled sacs, each containing one baby). Apart from being housed in the same uterus in this form of twinning each baby is independent of the other. Dizygotic twins always have this form of placentation. Monozygotic twins may have a dichorionic placenta if the single fertilised egg divides in the first 3 days after conception.

Monochorionic placentation (20% of twins)

Early monochorionic twin placentation

A monochorionic twin pregnancy means that there is one single placental disc with each baby sharing the placental mass with the co-twin. Inextricably dependent upon each other, this form of placentation can be viewed as conjoined twinning of the placenta.

This form of twinning means that both fetuses developed from the same fertilised egg, with division occurring more than 3 days after fertilisation.

Only 1% of monochorionic twin pregnancies will have a single sac: it is a very high risk pregnancy with a risk of entanglement of the umbilical cords of the two fetuses.

What are the major problems with multiple pregnancies?

The major complications of twin pregnancies include:

  1. Preterm birth (>50%)

  2. Intrauterine growth restriction

  3. Discordant growth profiles (where one twin is significantly larger than its co-twin)

  4. Discordant amniotic fluid volumes between the two fetuses

  5. Placenta previa (part or all of the placenta covers part or all of the lower uterine segment and cervix)

  6. Fetal structural anomalies

  7. Complications specific to monochorionic placentation (Twin-Twin Transfusion Syndrome, discordant structural anomalies, severe growth discrepancies)

As the complication rate for monochorionic twins is far greater than that for dichorionic twins your doctor will arrange for more frequent ultrasound surveillance in monochorionic twin pregnancies.

FAQs

How long will Multiple Pregnancy Ultrasounds take?

The duration of a twin pregnancy ultrasound can vary based on several factors, including the purpose of the ultrasound, the complexity of the examination, the positions of the babies, and the quality of images obtained. On average, a twin pregnancy ultrasound can take around 45 - 60 minutes.

As we need longer to do your scan, you will need to notify our reception team that you are booking for a twin pregnancy when making any appointments.

How often will I need ultrasounds during a twin pregnancy?

Twin pregnancies often require more frequent ultrasounds compared to singleton pregnancies. You might have ultrasounds every 4-6 weeks or more, depending on your healthcare provider's recommendations.

Can I find out the genders of my twins during ultrasound?

Yes, it's possible to determine the genders of twins through ultrasound, but this can be more challenging if the twins are positioned in a way that obstructs visibility.

You may need to wait until your 20 week scan to have the genders confirmed.

Can the position of the twins change over time?

Yes, the position of the twins can change throughout pregnancy. One twin might be head down while the other is breech, or they might both change positions multiple times.

Is there a difference between ultrasounds for twin pregnancies and singleton pregnancies?

While the basic principles of ultrasound imaging apply to both, ultrasounds for twin pregnancies often focus on monitoring the growth, positions, and potential complications of both twins..

Please ensure you tell our staff that you have a twin pregnancy when booking your ultrasound appointments.

What is twin-to-twin transfusion syndrome (TTTS) and how is it monitored through ultrasound?

TTTS is a condition in which there's an uneven blood flow between the twins through the shared placenta. Ultrasounds monitor blood flow and other signs of TTTS, allowing for timely intervention if needed.