Twins can be formed in two primary ways: identical (monozygotic) and fraternal (dizygotic).
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Identical Twins (Monozygotic): Identical twins occur when a single fertilized egg (zygote) splits into two separate embryos. This process usually happens within the first few days after fertilization. Since the twins originate from the same zygote, they share the same genetic material and are genetically identical, leading to similar physical characteristics and the same sex.
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Fraternal Twins (Dizygotic): Fraternal twins result from the fertilization of two separate eggs by two separate sperm cells. This can happen during a single ovulation cycle when a woman releases more than one egg. The resulting embryos will have different genetic compositions, as they come from different zygotes. Fraternal twins can be of the same sex or different sexes and may share some physical traits but do not have the same genetic makeup as identical twins.
Factors that can influence the likelihood of having twins include genetics, maternal age, number of previous pregnancies, and the use of fertility treatments.