A twin viability scan is an early pregnancy scan, usually carried out between 6 and 10 weeks gestation, which checks that a twin pregnancy is continuing normally and that both babies are developing inside the womb. It is useful in assessing early pregnancy viability by evaluating the number of embryos or fetuses and the cardiac activity when expected. It also determines the gestational age and, crucially in a twin pregnancy, it determines chorionicity and amnionicity.

The 2025 International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) guideline highlights that the first-trimester ultrasound examination is of utmost importance in twin pregnancy to assess chorionicity and amnionicity at an early stage of pregnancy.

What Is a Twin Viability Scan?

A Twin Viability Scan is an early pregnancy ultrasound test that determines the state of a twin pregnancy.

Depending on the gestational age of the pregnancy, the scan may be able to identify:

The number of pregnancies present

Whether the pregnancies are in the uterus or not

Gestational sacs

Yolk sacs, if any

Embryos, if any

Crown-rump length (CRL)

Cardiac activity, if any

Chorionicity

Amnionicity

The uterus and surrounding structures

The scan can identify these depending on the gestational age of the pregnancy. A pregnancy that is being examined too early may not have all of the above structures defined, and an inconclusive scan does not necessarily mean there is a complication.

Why Is a Twin Viability Scan Important?

A twin pregnancy can be classified into two different types based on the possibility that the fetuses may either have separate placentas and amniotic sacs or share a placenta. Therefore, an important aim of early ultrasonography in twin pregnancy is to assess chorionicity and amnionicity.

ISUOG recommends a determination of chorionicity before 13+6 weeks using intertwin membrane, its thickness, and placental masses. NICE states that a first-trimester ultrasound is recommended in twin pregnancies to estimate gestational age and establish chorionicity and amnionicity.

What can be seen in a Twin Viability Scan?

A Twin Viability Scan helps the doctor to take an early peek at both babies and the progress of the pregnancy. Depending on how far along the pregnancy is, a Twin Viability Scan can help to see:

1. Examination for two babies:

The two-baby check ultrasound scan allows determining whether a woman is carrying two infants. In particular, the two-egg conception test can detect two gestational sacs, two yolk sacs, two embryos, and two heartbeats in one pregnancy. The scan is considered a reliable test for conceiving twins or two separate pregnancies, depending on the stage of pregnancy.

2. Test for Examining the Location of Two Pregnancies

The second scan allows examination of whether both pregnancies are occurring in the uterus. It is performed when the woman experiences pain and bleeding, which indicate ectopic pregnancies.

3. Cardiac Activity:

As soon as the pregnancy is far enough along, the doctor can possibly see cardiac activity in both of the babies.

The doctor looks at each of the babies individually. When the scan does not show any cardiac activity in a very early scan, this does not necessarily mean that something is wrong, since some scans simply occur too early to see cardiac activity clearly, and your doctor may decide to reschedule the scan.

4. The baby's early growth:

The doctor will check the size and development of the fetuses to make sure they are growing as expected for how far along the pregnancy is. The physician may calculate the two fetuses’ growth based on Crown-Rump Length (CRL). These measurements will help the doctor determine the stage of the pregnancy in comparison to each of the fetuses’ initial sizes.

5. Whether the twins share a placenta:

One of the most crucial points of a scan in early stages is determining whether the babies share a placenta.

Twins can have:

Two placentas and two amniotic sacs: Dichorionic-Diamniotic (DCDA)

One placenta and two amniotic sacs: Monochorionic-Diamniotic (MCDA)

One placenta and one amniotic sac: Monochorionic-Monoamniotic (MCMA)

Being able to see this is important because different types of twin pregnancies require different amounts of care during the pregnancy.

6. Whether the babies share an amniotic sac:

The scan can also help to determine whether the babies have their own amniotic sacs or if they have to share an amniotic sac. This is called amnionicity.

Being able to see if they share a placenta or an amniotic sac helps the doctor to schedule the appropriate follow-up scans.

7. Identifying Twin A and Twin B:

The doctor may decide to identify the two babies as Twin A and Twin B, depending on their location in the uterus. This helps to identify the same baby during subsequent scans, in order to look after their progress as consistently as possible.

8. The early development of both babies:

The doctor can evaluate the early growth of both babies, depending on what can be seen at the time of the scan.

However, an early scan cannot detect every possible problem or assure that the babies will stay in perfect health throughout the pregnancy, since some structures can only be evaluated during later parts of the pregnancy.

What does "Chorionicity" mean??

Chorionicity describes about the placenta whether the twins share a placenta or have separate placentas. Twin pregnancies are broadly classified as:

Placenta:

The placenta can usually be called (Mavi). In some twin pregnancy cases, each baby has its own. The placenta is nothing but the temporary organ that connects the baby through the umbilical cord; It gives the baby food and oxygen, takes away waste, and makes important hormones.

When both babies have shared placentas, each one has its own; it can be called “Dichorionic Twin Pregnancy.”

When two babies share only one placenta, it can be called “Monochorionic Twin Pregnancy.”

What does "Amnionicity " mean??

The term Amnionicity means whether the babies have their own amniotic sacs or if they have to share an amniotic sac. This is called amnionicity.

After a complete checkup of the placenta, the doctor examines the membrane arrangement to determine how many amniotic sacs are present. An amniotic sac is a kind of thin, tough, transparent pair of membranes (the inner amnion and outer chorion) inside the uterus that surrounds, protects, and supports a developing fetus during pregnancy; the Amniotic sac is filled with amniotic fluid. The amniotic fluid cushions the baby and protects the baby in the mother's womb.

Types of amniotic sacs:

MCDA - Monochorionic Diamniotic

The “Monochorionic Diamniotic” means that the twins are identical twins who share one placenta but live in two separate amniotic sacs. In simple terms, the babies share one placenta, but each baby has its own separate amniotic sac.

MCMA - Monochorionic Monoamniotic

The “Monochorionic Monoamniotic” means that the twins are identical twins who share one common placenta and one single amniotic sac without a dividing membrane. It accounts for about 1% of all twin pregnancies, so it requires specialist assessment and closer monitoring.

Conclusion:

A Twin Viability Scan is a test that helps doctors determine the stage of development of both babies and learn more details about the twin pregnancy. Establishing the type of twin pregnancy is crucial for receiving proper care and monitoring from healthcare providers. In case the scan was taken during the first stages of pregnancy and it was not possible to detect all the necessary details, the doctor may recommend a scan that can provide a more detailed examination.

Book your First Viability Scan with Naren Ultrasound & Fetal Medicine Center and take this important step with expert guidance and confidence.

About the Doctor

Dr. Naren Satya

Consultant Radiologist

Naren Ultrasound and Fetal Medicine Center, Visakhapatnam