Twin pregnancy is a physiological event characterized by a gestation in which more than one fetus develops in the uterus.
Several factors predispose to twin pregnancy: maternal age over 35, maternal heredity, nationality (it is far more common in African countries than in Eastern ones), and fertility treatment techniques.
In the human species, twin pregnancy is a relatively uncommon event: a twin pregnancy occurs in one out of every 80 pregnancies, and a triplet pregnancy in one out of every 800.
Since 1975, the incidence of multiple pregnancies has increased due to fertility treatment techniques; indeed, ovulation induction and the various methods of in vitro fertilization result in multiple pregnancies in 10 to 35% of cases.
Twins can be distinguished based on their origin (that is, how fertilization occurred) and can be identical (monozygotic) or fraternal (dizygotic, meaning siblings born at the same time).
Monozygotic pregnancy
Monozygotic pregnancy (which accounts for a third of twin pregnancies) results from a single sperm fertilizing a single egg; it is the result of pure biological chance and is not influenced by external factors.
The fertilized cell splits into two parts, giving rise to two embryos with the same genetic makeup, and therefore the same sex and identical physical characteristics.
If the egg separates early, that is, within the first four days, two amniotic sacs and two placentas form. This type of pregnancy is called «dichorionic and diamniotic».
If separation occurs later, between the fourth and eighth day, two amniotic sacs form but only one placenta. This type of pregnancy is called «monochorionic and diamniotic».
If separation occurs even later, after the eighth day, only one amniotic sac and one placenta form. This type of pregnancy is called monochorionic and monoamniotic.
After the thirteenth day, conjoined, or Siamese, twins form. This is a very rare event, in which the organs and tissues shared by the two embryos show severe malformations, which are often the cause of premature death, including in utero.
Dizygotic pregnancy
Dizygotic pregnancy (which accounts for two thirds of twin pregnancies) results from two sperm fertilizing two separate eggs. It is always dichorionic and diamniotic, and today it is often the result of fertility treatment involving superovulation and the production of multiple follicles.
In Italy today, around 5,600 twin pregnancies and around 280 triplet pregnancies are recorded each year. Ultrasound performed at 7 to 8 weeks of gestation makes it possible to diagnose a multiple pregnancy.
Complications of twin pregnancies
In the first trimester, the greater rise in the pregnancy hormone beta-hCG is responsible for more pronounced digestive symptoms, with increased nausea and vomiting.
There may be a higher risk of gestational diabetes, hypertension, and anemia.
In the last trimester, the increased size of the uterus is responsible for a higher incidence of fluid retention, varicose veins, and hemorrhoids.
Weight gain is naturally greater, as is fatigue.
The risks for the fetuses include miscarriage, mainly linked to the fact that the uterus is not designed to carry two fetuses. Implantation defects and severe embryonic malformations are the most frequent cause of miscarriage; growth restriction, which is very common and can affect both twins or cause a discrepancy in growth between the two fetuses; and preterm birth.
Fetal presentations are often incompatible with natural birth, which is only possible when the fetuses are in a vertex presentation and there are no maternal or fetal conditions present. Cesarean section is therefore the most frequently chosen option in twin pregnancies.
In Latin, gemellus means «double»: two individuals born during the same birth but at different moments.
In English, the word twins comes from an old Anglo-Saxon word referring to two strands of wool so tightly intertwined that they cannot be separated.
The birth of twins has always been considered a remarkable event that challenges the sense of individual uniqueness, arousing interest, fascination, repulsion and curiosity, whether because of the unusual appearance that makes them look more like fetuses than newborns (their birth is often premature), or because of the malformations that occur more frequently than in single births.
Depending on the culture, twin births are viewed in different ways, ranging from veneration to deep aversion. In the East, twins are considered supernatural beings sharing a single soul, worshipped as a divine gift but often also as objects of sexual attraction, even of pedophilia. In many African countries, on the other hand, they are considered harbingers of misfortune, because of the risks they entail.