There are two types of twin pregnancies, identical or monozygotic twins and fraternal or dizygotic twins. Identical twins are conceived from a single egg that has been fertilized by a sperm. During the initial stage of development, the fertilized egg splits into two or more parts, each forming a separate embryo. Identical twins share a common placenta, but have separate amniotic sacs. In very rare cases, a single amniotic sac is shared by identical twins. These are known as monoamniotic twins. Fraternal twins are formed when two or more eggs undergo fertilization by different sperm.
These twins have their own placentas and amniotic sacs. Types of multiple births also include conjoined twins. This occurs when the zygote splits very late.
Conjoined twins may share certain organs and body parts, apart from a common placenta and amniotic sac. Conjoined twins are also referred to as Siamese twins. Given below are some features of both types of twin pregnancy.
Identical twins are always of the same sex and have the same blood type. They may not always look alike. Also, it is possible that one twin may be right-handed while the other is left-handed. Identical twins are not the result of hereditary factors, race or maternal age. They can develop in any pregnancy. The twins that result from fertility treatments are usually fraternal. Fraternal twins or multiples can be of different sexes and also have different blood types. They often look quite different from each other. There is much variation in appearance and features and fraternal twins often have different colored eyes and hair. Some fraternal twins may, however, look alike as many siblings do. Fraternal twins are more likely to develop when there has been a family history of fraternal twin pregnancies. They occur more commonly in mothers who are older. African races are observed to experience fraternal twin pregnancies more commonly. Women who have already carried fraternal twins are more likely to become pregnant with twins in the future. More information on twin pregnancy week by week
There are some common causes that lead to different types of twin pregnancy. A higher maternal age indicates a higher possibility of having twins. Women who discontinue birth control pills are more likely to become pregnant with twins in the initial few menstrual cycles. Fertility treatments are aimed at stimulating the release of multiple eggs from the ovaries. Therefore women who undergo such treatment are more likely to have twin or multiple pregnancies.
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