Understanding Types and Diagnosis of Twin Pregnancies

Various Types of Twins

A) Fraternal Twins (Dizygotic) (70%)

Fraternal twins result from two different eggs, each fertilized by a different sperm. They may or may not be of the same sex, and their blood groups can be equal or different.

Superfecundation

This occurs when, during a single cycle, one ovary simultaneously releases two oocytes. Each oocyte can be fertilized by sperm from different acts of intercourse.

Superfetation

This is very exceptional, where the two oocytes come from two consecutive cycles. Implantation occurs at two widely separated points with separate placentas.

B) Identical Twins (Monozygotic) (30%)

Identical twins arise from the division of a single egg.

1. Separation in the Blastomere Phase

This results in dichlorionic diamniotic twins. The first two blastomeres separate from each other, and each develops independently. Consequently, each twin has its own placenta, amnion, and chorion. In these cases, identity as identical twins is confirmed by genetic testing.

2. Separation at the Embryonic Bud Stage

This results in monochorionic diamniotic twins. There is a single blastula with a common trophoblastic shell for both embryonic buds, meaning there is only one placenta and one chorion, but two separate amnions.

3. Separation at the Embryonic Disc Stage

The appearance of two distinct lines in the early embryonic disc, after the amniotic cavity has formed, results in monochorionic-monoamniotic twins.

Etiological Conditions and Frequency

The frequency of twin births increases with maternal age and especially with parity (mainly for dizygotic twins). The role of heredity is undeniable (especially for dizygotic twins). Finally, the frequency of twin pregnancies appears to decrease from colder regions to warmer countries.

Trouble Diagnoses

A) Twin Pregnancy Diagnosis

The diagnosis of twin pregnancy can only be reliably established after the 2nd trimester, often becoming obvious around the 6th month when the uterus is noticeably bulky beyond the expected size. Definitive diagnosis is established from the beginning of the 3rd quarter by X-ray, which shows the general shape of two fetal skeletons, or earlier, during the 2nd quarter, by ultrasonography.

Morbidity and mortality for the fetuses are increased in twin pregnancies compared to singleton pregnancies, primarily due to prematurity and low birth weight. For these reasons, twin pregnancies require more careful monitoring.

B) Diagnosis of True Versus False Twins

If the newborns are of different sexes, they are obligatorily fraternal (false) twins.

• If they are the same sex and there are indisputable placental vascular anastomoses, they are monozygotic (identical) twins.
• If they are the same sex, but the existence of placental vascular anastomoses cannot be confirmed, it is necessary to determine the nature (monozygotic or dizygotic) by studying hereditary traits such as blood groups and dermatoglyphics. Indeed, not all dichorionic twins are dizygotic.