How do fraternal twins form?

Fraternal twins, also known as dizygotic or non-identical twins, occur when two separate eggs are fertilized by two different sperm within the same menstrual cycle.

1. Two Eggs: During ovulation, instead of releasing a single egg, the ovaries release two mature eggs. This can occur due to a surge in hormones or genetic factors.

2. Fertilization: Each of the two eggs is fertilized by a separate sperm cell in the fallopian tubes, resulting in two distinct embryos. Since they develop from separate eggs and sperm, fraternal twins can be of the same or different sexes and have distinct physical features and genetic makeups.

3. Implantation: The fertilized embryos then attach (implant) to the uterine wall and develop independently, each within its own amniotic sac and placenta.

4. Pregnancy: The resulting pregnancy involves carrying two separate fetuses, similar to regular pregnancies with singleton babies. Each twin grows and develops individually throughout the gestation period.

5. Birth: Fraternal twins can be delivered simultaneously (at the same time) or with a short interval between their births. The timing of birth depends on various factors, including the mother's health and the position of the babies.

In summary, fraternal twins occur due to the fertilization of two separate eggs, leading to the development of distinct embryos and individual placentas. They are not genetically identical like identical twins, and their physical characteristics and genetic traits can differ. They share the same environment in the uterus but have independent genetic blueprints.