Guide to twin pregnancy: characteristics, tests and delivery

When there are two fetuses, one or two placentas can form and, as Dr. Marinella D’avanzo explains, this situation is called chorionicity . She still tells us:

“If there are one or two placentas, the conduct of pregnancy radically changes. Chorionicity is visualized in the first trimester of pregnancy with sonographers accustomed to doing ultrasounds OF twin pregnancies. 

Depending on the number of placentas, pregnancies are defined as bichorionic or monochorionic.

• Bichorial pregnancy shows a typical sign on ultrasound, called a lambda, which is seen between 8-9 weeks of pregnancy.
• Monochorionic pregnancy, on the other hand, shows what is called the T sign.

Distinguishing between mono and bichorionic is critical to the pregnancy journey and this distinction can only be made by an experienced sonographer.”

We will see later what it means to follow a different pregnancy path depending on the chorionicity in multiple pregnancy.

How many amniotic sacs are there?

The amniotic sac is a membrane that surrounds and protects the embryo.

In a twin pregnancy, there are two or more embryos, but there may be only one amniotic sac containing both, or one for each embryo.

The doctor explains: “The number of amniotic sacs present is medically called amnionicity . If there are two, it is a diamniotic pregnancy. If there are 3 bags it is a triamniotic pregnancy and so on. If there is only one amniotic sac, it is called a monoamniotic pregnancy.

Same or different twins?

We know that twins means having developed during the same pregnancy, but there are twins who are born identical and others who are born different (not only physically but also of different sexes).

Dell’Avanzo explains: “In more precise terms, it must be said that twins are homozygous or heterozygous. This fact is called zygosity .

• Homozygous twins (from “homo”, identical): a single egg has been fertilized by a single sperm and gives rise to two identical individuals, i.e. with the same genetic heritage. Phenotypic discordance is rare.
• Heterozygous twins (from “hetero”, different): they derive from two different ovules fertilized by two different sperm cells”

The twin pregnancy therefore has these three biological characteristics: chorionicity, amnionicity and zygosity. Different combinations can occur which, as mentioned, involve a different approach to pregnancy both from the point of view of the mother and from the medical point of view.

Let us then explore the various paths of twin pregnancy.

The journey of twin pregnancy

The management of a twin pregnancy depends on the type of pregnancy.

Dr. Marinella Dell’Avanzo explains: “there is a clear distinction between bichorionic and monochorionic pregnancies. The bichorial-diamniotic pregnancy is the most frequent twin pregnancy ever and it is also the one that involves the least risks, but in any case it must be followed by an expert operator.

The bichorionic-diamniotic twin pregnancy

We have seen that bichorial pregnancy has two placentas and diamniotic pregnancy two amniotic sacs . There are therefore two distinct individuals : either two males or two females or one male or one female.

The first trimester should be managed like a singleton pregnancy .

Subsequently, according to the guidelines of the SIEOG  (Italian Society of Obstetrics and Gynecology Ultrasound) the pregnancy should be checked every 4 weeks .

This is because from the 20th week additional risks may arise which are: the restriction of the growth of a twin or the threat of preterm birth.

Being distinct individuals there is no additional risk of malformation and heart disease and therefore they are considered as single fetuses. If a twin is missing, the pregnancy proceeds as a single.

Monochorionic twin pregnancy

Monochorionic pregnancy has only one placenta for both (or more) fetuses.

For this type of pregnancy the path is completely different . It must be managed by experts in monochorionic pregnancies.

The twins share a single placenta and are identical twins (either two males or two females, it is extremely rare to find monochorionic twins with discordant phenotype).

In a monochorionic pregnancy we have two special cases: diamniotic (the fetuses are in separate sacs) or monoamniotic (the fetuses are in the same sac).

Diamniotic monochorionic pregnancies

The twins are each in their own sac but share the same placenta.

These are pregnancies that must be constantly monitored from the 16th week to the 36th week every 2 weeks because there is a risk of fetal transfusion from one twin to the other . In simple words there is an imbalance between two fetuses one of which becomes the recipient and the other the donor.

Understanding that a feto-fetal transfusion is taking place is essential to be able to intervene promptly. There are 4 stages of evaluation of the severity of the feto-fetal transfusion which occurs with an incidence of 10%.

The therapy consists in the laser closure of the vascular anastomoses that form in the placenta.

When the doctor sees an initial feto-fetal transfusion, he must refer to a reference center that does laser therapy to have adequate treatment. It is important to know that if a twin is missing in monochorionic pregnancies, the remaining co-twin could suffer neurological damage which will then be evaluated with possible magnetic resonance imaging.

Monochorionic monoamniotic pregnancies

They are very rare pregnancies (1% of monozygotic pregnancies). The twins are in the same amniotic sac and share a single placenta .

This type of pregnancy has a high risk of stillbirth because the umbilical cords become entangled .

Pregnancy monitoring must therefore be intensive : from 16 weeks onwards controls must be every week .

At each checkup, we evaluate how the twins are growing, their vitality, the flows and the state of the umbilical cords and any twisting of the umbilical cords.

Usually these pregnancies end in a caesarean section before the 32nd week.

Prenatal counseling for twin pregnancies

We have seen all the physiological characteristics of twin pregnancies and how they are very different depending on the type.

Proper prenatal counseling is therefore essential in twin pregnancies and must be done after having identified exactly the type of multiple pregnancy in progress.

Once the typology has been confirmed, the gynecologist must present the various options to the couple (combined test, fetal DNA analysis, through multidisciplinary counseling.

As explained, bichorionics have fewer risks while monochorionics have more risks of both transfusions and chromosomal abnormalities. Counseling must therefore be clear and must take into account all available options. It is important in counseling to know if the pregnancy derives from an egg donation.

The thickness of the nape or nuchal translucency

In twin pregnancies, regardless of the tests that will be done, it is always good to evaluate the thickness of the neck at 12 weeks of pregnancy.

It has been seen that in the case of monochorionic pregnancy a thick nape may indicate a possible future feto-fetal transfusion.

In the case of bichorials, if a twin has a thicker nape he is the one who can be more at risk. If you opt for invasive diagnosis, this must be precise: you need to know perfectly from which twin you take the sample to be analyzed and identify it correctly for any subsequent selection.

Amniocentesis and CVS in twin pregnancies

Amniocentesis and CVS are invasive prenatal tests.

In bichorials both amniocentesis and CVS can be performed bearing in mind that there will be two needle insertions and therefore the risk of miscarriage will slightly increase.

In monochorionics, only one needle insertion can be done but one must be sure that there are no discordant factors, such as growth restriction of a twin or increased translucency. In these cases the withdrawals must be two.

Delivery times in twin pregnancies

Diamniotic bichorionic pregnancies are those with the least risk factors: they can reach 37-38 weeks and can also undergo natural birth .

Monochorionic diamniotic pregnancies (with two amniotic sacs) usually end with a cesarean section around the 36th week.

It is preferable to proceed with the cesarean section because a feto-fetal transfusion can take place during labor.

Monoamniotic monochorionics usually end with cesarean section before the 32nd week of pregnancy.

In summary, childbirth for a multiple pregnancy takes place:

• Around the 37-38th week also in a natural way for pregnancies with two placentas and two amniotic sacs
• Around 36 weeks of pregnancy by cesarean section for pregnancies with only one placenta and two amniotic sacs
• Around the 32nd week of pregnancy for the very rare with only one placenta and only one amniotic sac, always with cesarean section

The most common problems

The mother expecting twins may suffer more from nausea and gastric disorders and may be more likely to encounter gestational diabetes [SIEOG] or hypertension .

Due to the increased weight of the uterus, it is more subject to shortening of the uterine cervix and for this reason close checks are very important to evaluate all the parameters of the cervix using transvaginal ultrasound.