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How do Twins Form? Who is More Likely to Have a Twin Pregnancy? What are the Implications of a Twin Pregnancy?
In the past, multiple pregnancies were considered exceptionally rare. Nowadays, due to an increase in maternal age and the widespread use of various fertility methods, including fertility drugs and in vitro fertilization, twin births (and more) have become much more common, accounting for approximately 2.4% of all births in Israel.
A singleton pregnancy begins when a sperm fertilizes an egg, resulting in a zygote. Over the subsequent days, the fertilized egg, or zygote, undergoes multiple divisions to form the cell mass that will become the fetus. This cell mass implants in the uterine wall and develops into a single baby.
Identical twins also originate from a single fertilized egg. During the first week following fertilization, the single fertilized egg divides into two, and these cells continue to divide and implant in the uterine wall. Identical twins share the same genetic material and are of the same sex, constituting about one-third of all twin births.
Fraternal twins develop from two separate eggs fertilized by two different sperm cells. Typically, only one egg is released during ovulation, but in the case of fraternal twins, two eggs are released and fertilized by two separate sperm cells. Two separate embryos develop, each with its placenta and amniotic sac. Fraternal twins carry different genetic loads and may be of the same sex or different sexes, similar to other siblings.
While a typical pregnancy lasts for 40 weeks, twin pregnancies usually last no longer than 38 weeks (with an average of 36 weeks). This shorter duration is often attributed to the increased demands on the mother's body and the difficulty in providing for the needs of both fetuses within the womb. In many cases, twins are born prematurely and may have lower birth weights and potential complications due to their prematurity.
During the first ultrasound examination of the pregnancy, typically around the sixth week, both sacs of a twin pregnancy can be distinguished. By around the 10th week, the number of amniotic sacs can be identified, helping determine whether the pregnancy involves identical twins or fraternal twins.
Twin pregnancies are considered more complex than singleton pregnancies and require closer monitoring, including more frequent check-ups.
The mode of delivery for twins depends primarily on the presentation of the first fetus. When the first fetus presents headfirst, a vaginal delivery is possible.
However, in many cases of twin pregnancies, a cesarean section (C-section) is planned and anticipated to ensure the safe delivery of both infants.
The average time between the birth of the first twin and the second twin is approximately 20 minutes.
