Twin ultrasound: how and when it should be done

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Twin ultrasound

The incidence of twin pregnancies is increasing, mainly due to advanced maternal age and the consequent use of assisted reproductive techniques. And with the increase in multiple pregnancies, it has also become necessary to update the guidelines relating to the management of this type of pregnancy, including ultrasound. So let's see together how thetwin ultrasound and what is it for.

In this article

  • Ultrasound: when twins are seen
  • Symptoms of twin pregnancy
  • Zygotic or monozygotic twins
  • Nuclear translucency for twins
  • How many ultrasound scans need to be done?

Ultrasound: when twins are seen

Generally it turns out to expect two or more twins on the occasion of the first ultrasound which is performed around the fifth / sixth week of pregnancy (although it is advisable to wait until the tenth week to get a clearer picture). At that time the operator will evaluate, in addition to the number of embryos:

  • that the baby's heart is beating and with a correct rhythm
  • the volume of amniotic fluid
  • where the placenta is located within the uterus.

Symptoms of twin pregnancy

Many women expecting twins experience fairly obvious and very early pregnancy symptoms, including:

  • fatigue,
  • emotional changes,
  • nausea and vomit,
  • constipation.

Furthermore, the body changes with a twin pregnancy they are much more evident than with a single pregnancy. If you are pregnant with twins, you could gain 16-20 kg (compared to 10-15 kg in a single pregnancy) and at the beginning of pregnancy the levels of Beta Hcg hormones rise more rapidly and exponentially.

Zygotic or monozygotic twins

L'echography it also serves to understand if the twins are heterozygous (also called "twin brothers" or monozygotic, therefore identical.

  • I dizygotic twins derive from the fertilization of two oocytes: each fetus, therefore, is different from the other and also has its own specific risk of chromosomal anomaly.
  • I monozygotic twins derive from the fertilization of a single oocyte and subsequent division of the zygote. The timing in which the division of the zygote will take place will determine the type of placentation, that is the chorionicity: bicorionic ‑ biamniotic, monochorionic ‑ biamniotic or monochorionic ‑ monoamniotic pregnancy up to conjoined twins.
Read also: Expecting twins

Nuclear translucency for twins

one of the screening methods that can be carried out in the first quarter is the Bitest, which involves two steps:

  • ultrasound for the evaluation of fetal anatomy, nuchal translucency and other "markers" of any anomalies;
  • a sample of maternal blood to make the determination of free β-hCG and PAPP-A.

These tests are used to calculate a percentage of the probability that the fetus has Down syndrome. It is not a genetic test and it does not analyze amniotic fluid like theamniocentesis, and therefore it is not a diagnostic test. It only serves to make a calculation of the probability that the child has Down syndrome and in the event of a positive result the expectant mother is advised to undergo amniocentesis.

According to experts it is combined screening of the first trimester, measurement of nuchal translucency and biochemical dosage in maternal blood, has a lower performance in twin pregnancy than in single pregnancy, but is nevertheless effective, with an identification capacity of about 89% and 5.4% of false positives.

How many ultrasound scans need to be done?

According to the SIGO guidelines, clinical monitoring of multiple pregnancy must be carried out by an obstetrician-gynecologist with good knowledge of the peculiarities of this type of pregnancy.

For biamniotic biamniotic pregnancies "BCBA", a monthly clinical check-up is recommended, which in the third trimester, due to the increased risk of obstetric complications, should be closer.

An ultrasound examination should also be done every month, including the estimate of fetal weights, and this frequency should be intensified where one emerges growth discrepancy between the twins exceeding 20-25%.


  • ISUOG Practice Guidelines: role of ultrasound in twin pregnancy
  • Multiple pregnancy management - SIGO
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