A girl gets pregnant with twins in two primary ways: either two separate eggs are fertilized by two separate sperm, or a single fertilized egg splits into two separate embryos.
Types of Twins
It's important to understand the different types of twins to understand the mechanisms involved.
- Fraternal Twins (Dizygotic): These twins develop from two separate eggs fertilized by two separate sperm. They are genetically distinct, just like any other siblings, and can be the same or different sexes.
- Identical Twins (Monozygotic): These twins develop from a single fertilized egg that splits into two separate embryos. They share nearly identical DNA and are always the same sex.
Mechanisms Leading to Twin Pregnancies
1. Fraternal Twins: Multiple Ovulation
The most common way for a girl to get pregnant with twins is through the fertilization of two separate eggs. This is known as hyperovulation, meaning the ovaries release more than one egg during ovulation.
- Hormonal Stimulation: Certain fertility treatments and procedures stimulate the ovaries, increasing the likelihood of releasing multiple eggs.
- Genetics: A family history of fraternal twins increases the chances of a woman releasing multiple eggs during ovulation. The tendency for hyperovulation can be inherited.
- Age: Women over 30 are more likely to release multiple eggs during ovulation than younger women.
- Ethnicity: Certain ethnicities have a higher incidence of fraternal twins.
- Parity: Women who have already had children are slightly more likely to conceive fraternal twins.
If two eggs are released and both are fertilized by separate sperm, fraternal twins result. Because they originated from different eggs and sperm, they are no more alike than any other siblings.
2. Identical Twins: Embryo Splitting
Identical twins occur when a single fertilized egg splits into two separate embryos very early in development. The exact cause of this split is still unknown, but it is generally considered a random event.
- Spontaneous Event: Unlike fraternal twins, there are no proven links to genetics, age, race, or fertility treatments for identical twins. It seems to occur randomly.
The timing of the split determines whether the twins share a placenta and amniotic sac. In rare cases, the split can be incomplete, resulting in conjoined twins.
Summary
Twin pregnancies result from two main pathways: the fertilization of two separate eggs (fraternal twins) or the splitting of a single fertilized egg into two embryos (identical twins). Fraternal twins are influenced by factors like fertility treatments, genetics, age, ethnicity, and parity, while identical twins are generally considered a spontaneous occurrence.
