• Clinical science

Multiple pregnancy


Multiple pregnancy refers to pregnancy with two or more fetuses. Twin pregnancies can be differentiated into monozygotic and dizygotic pregnancies. Monozygotic twin pregnancies result from the division of the fertilized oocyte into two embryonic layers, whereas dizygotic twin pregnancies arise from the fertilization of two oocytes with two spermatozoa. Multiple pregnancies are classified based on how the amniotic sac and placenta are divided among the fetuses, which is determined via ultrasound. In monozygotic twin pregnancies with a shared placenta (monochorionic), twin-to-twin transfusion syndrome is a possible complication. In these cases, blood vessels are shared between the twins, with one twin transferring blood (donor) to the other twin (recipient). Twin-to-twin transfusion syndrome is associated with various risks, including anemia and growth retardation in the donor twin, and polycythemia in the recipient twin. Pregnancies with more than one fetus are generally considered high-risk pregnancies, increasing the likelihood of almost all potential complications of pregnancy, including preterm labor, pregnancy-induced hypertension, and preeclampsia.


  • The frequency of multiple pregnancies (usually ∼ 1:89 pregnancies) has increased, which is attributed to the increase in in vitro fertilization.


Epidemiological data refers to the US, unless otherwise specified.


  • Predisposing factors
    • Advanced maternal age (> 35 years)
    • Previous multiple pregnancy
    • Use of fertility enhancing treatments
    • Maternal family history of dizygotic twins



Monozygotic vs. dizygotic twins

Identical twins (monozygotic twins) Fraternal twins (dizygotic twins)
Frequency ⅓ of all twin pregnancies ⅔ of all twin pregnancies
Origin Division of the fertilized oocyte into two embryonic layers Fertilization of two oocytes with two spermatozoa
Genetics of the individual Genetically identical Genetically different
Chorionic cavity and amniotic sack Varies (see below) Dichorionic-diamniotic

Special features in the development of monozygotic twins

A dizygotic pregnancy results in a dichorionic-diamniotic pregnancy. In monozygotic pregnancies, there are various ways in which the amniotic sac and placenta are shared.

Description Frequency in monozygotic twins
Dichorionic-diamniotic Each twin has an individual amniotic sac and placenta. ∼ 20–30%
Monochorionic-diamniotic The twins share a placenta and have individual amniotic sacs. ∼ 70%

The twins share both the placenta and amniotic sac.

∼ 1–5%
Monochorionic-monoamniotic (conjoined twins) The twins share the placenta and amniotic sac, and are conjoined. < 0.1%



Physical examination

  • Fundal height and abdominal girth are unusually large for the gestational age.
  • The presence of more than one fetus may, in some cases, be confirmed by palpation.
  • Two or more fetal heart rates can be heard on auscultation.


  • Evidence of more than one fetus
  • Differentiation between monochorionic and dichorionic twins during early pregnancy
    • Dichorionic twins: lambda sign
    • Monochorionic twins: T-sign

Prenatal care

Multifetal pregnancies are high-risk pregnancies and require more frequent prenatal care visits.

  • Frequent early evaluations for monochorionic twins to assess for twin-twin transfusion syndrome
  • From the 32nd week of pregnancy: weekly prenatal care visits, including ultrasound, to monitor fetal growth



There are various complications associated with multifetal pregnancies. Almost all complications associated with normal pregnancies are more likely in multifetal pregnancies.

Maternal illnesses

Fetal illnesses

  • Spontaneous reduction, or vanishing twin syndrome, can occur during the first trimester.
  • Twin-to-twin transfusion syndrome
    • In monochorionic twin pregnancies, the twins share a placenta. Blood flowing in a fixed direction from one twin to the other results in the transfer of blood from the donor twin to the recipient twin. This poses a risk to both fetuses.
  • Increased risk of neonatal morbidity (growth restrictions, congenital abnormalities) and mortality


We list the most important complications. The selection is not exhaustive.