When you have a double pregnancy, two situations can occur:
- That a double ovulation has occurred during the cycle, and two sperm fertilize those two eggs. This will result in dizygotic twins.
- That there is only one fertilized egg, but that it divides in two in the first weeks of gestation, giving rise to two individuals. This will result in monozygotic or identical twins.
Monozygotic or identical twins
Identical twins, also called monozygotic twins, arise shortly after fertilization. Once the sperm has fertilized the egg, during the first two weeks of pregnancy, it divides into two forming two individuals with the same genetic material.
Coming from the same zygote they share practically 100% of the genetic material. For this reason, identical twins will have the same physical appearance and gender. However, it must be beared in mind that there are many environmental influences that can change the characteristics of these identical twins
Dizygotic or fraternal twins
On the contrary, in the same pregnancy two embryos can be gestated from two eggs that have been fertilized by two spermatozoa. Each egg evolves separately so that each embryo will have its own amniotic sac and its own placenta. This is the case with dizygotic twins or fraternal twins. These individuals share the same genetic load as any other sibling of mother and father, who share about 50%, but are not identical, so they may be of different gender and appearance, and have different characteristics.
Multiple pregnancies have increased in recent years due to fertility clinics, which are improving their techniques, and the age of mothers, which with the delay of motherhood and due to hormonal changes and proximity to menopause, the ovulation of more than one egg increases and therefore the possibility of double pregnancy.
While it is true that assisted reproduction increases the probability of multiple pregnancy, according to data collected by the European Society of Human Reproduction and Embryology (ESHRE) more than 73% of pregnancies are unique, compared to 24% of twins.