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 births is calculated in accordance with Hellin's law. 
- Twins: ∼ 1:89
- Triplets: ∼ 1:892 (1:7,921)
- Quadruplets: ∼ 1:893 (1:704,969)
- 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 
Monozygotic vs. dizygotic twins
|Comparison of monozygotic vs. dizygotic twins|
|Identical twins (monozygotic twins)||Fraternal twins (dizygotic twins)|
|Frequency|| || |
|Genetics of the individual|| || |
|Chorionic cavity and amniotic sack|| |
Special features in the development of monozygotic twins
- Dizygotic pregnancy results in a dichorionic-diamniotic pregnancy.
- In monozygotic pregnancies, there are various ways in which the amniotic sac and placenta are shared.
|Overview of different types of monozygotic twin pregnancy|
|Description||Time of division of the zygote||Frequency in monozygotic twins|
|Dichorionic-diamniotic|| || |
|Monochorionic-diamniotic|| || |
|Monochorionic-monoamniotic|| || |
Monochorionic-monoamniotic (conjoined twins)
| || |
A four-wheeler has SPACe for twins.” 1st four days (0–3): Separate placenta and amniotic sac; 2nd four days (4–7): shared Placenta; 3rd four days (8–11): shared Amniotic sac; day 12 and beyond: Conjoined twins.
- Fundal height and abdominal girth are unusually large for the gestational age.
- 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-to-twin transfusion syndrome
- From the 32nd week of pregnancy: weekly prenatal care visits, including ultrasound, to monitor fetal growth
Maternal illnesses 
- Preterm labor and birth (most common complication)
- Hyperemesis gravidarum
- Gestational diabetes
- Preeclampsia, eclampsia, pregnancy-induced hypertension
- Cervical incompetence, premature rupture of membranes
- , hypotrophy, and intrauterine malnutrition of at least one fetus
- Uterine atony
- Miscarriage or loss of one fetus during the first trimester
- Birth complications: prolonged first stage of labor; , premature placental abruption after birth of the first fetus, prolapsed cord
- Increased risk of maternal morbidity
- Spontaneous reduction or vanishing twin syndrome: can occur during the first trimester
Twin-to-twin transfusion syndrome 
- Affects 10–15% of monochorionic twin pregnancies (twins that share the placenta)
- Blood is continuously shunted from one twin to the other through vascular anastomoses on the shared placenta, posing a risk to both fetuses.
- Recipient twin
- Donor twin
- Cord entanglement: can occur only in monoamniotic twin pregnancies
- Increased risk of neonatal morbidity; (growth restrictions, congenital abnormalities) and mortality
We list the most important complications. The selection is not exhaustive.