PREGNANCY-BIRTH Messages for November, 1999: Re: Solve a disagreement. . . identical or fraternal

Re: Solve a disagreement. . . identical or fraternal

From: Christine

anonymous@obgyn.net

Thu, 11 Nov 1999 12:27:12 -0600 (CST)

Types of Twins Monozygotic or Identical: Monozygotic or MZ twins derive from the division of a single zygote(fertilized ovum) during the first developing stages of the embryo after fertilization, hence the term monozygotic. These separate cell masses become embryos which are genetically identical and will be the same sex. The cell division happens within the first week of fertilization. Each MZ twins usually has her own amniotic, or inner, membrane. Sixty percent of twins have their own amniotic sacs and are contained within the same chorionic, or outer, membrane. However, like fraternal twins, MZ twins can have separate amniotic and chorionic sacs, and placentas. Separate amniotic and chorionic sacs with separate placentas occurs in about 25 percent of MZ twins. One to 2% of MZ twins share both amniotic and chorionic sacs and placenta. MZ twins have similar if not the same blood groupings.

Dizygotic or Fraternal Twins Dizygotic or DZ/fraternal twins derive from the fertilization of two independently released ova, hence the di in dizygotic. They will be no more genetically alike than ordinary brothers and sisters. Fraternal twins can be same sex or boy-girl. Egg pairs occur when there are multiple ovulations during the same ovulation cycle. Each egg is fertilized singly and develops singly.. DZ twins usually have their own inner and outer sacs, but can "share" placentas during instances when the placentas fuse. There are even instances two eggs being fertilized by two different sperm. Women have delivered twins fathered by two different men, as genetic tests have proven.

Conjoined or "Siamese" Twins Incomplete or late division of the zygote and subsequent cell masses can result in conjoined twins. One in 50,00-80,000 births results in conjoined twins. See also the Zygote conjoined twins information page at Swarthmore University for further reading.

"Mirror Image" Twins Possibly twins who separated later in development since, like conjoined twins, they show signs of "reversed asymmetrical physical characteristics." This "mirror effect" shows up in handedness, hair whorls, tooth development, birthmarks. Scientists also note that mirror and conjoined twins can be "opposites" personality-wise too. Mirror imaging occurs in about 25% of MZ births.

Disappearing Twin The discovery of the phenomenon of the "disappearing twin" is a late 20th Century one, due to technological advances which allow doctors to see inside a woman's uterus. Often a pregnancy will be diagnosed as a twin pregnancy, but when the woman gives birth, there is only one child. Scientists theorize that the other twin was absorbed by the surviving twin, or was aborted mid-pregnancy due to malnourishment. In rare moments, a baby will be born with her sister "still attached" as a cyst, an extra limb, or as more internal organs than is normal. See also Conjoined Twins.

Multiples or Supertwins More than a pair born at once. Combinations can include monozygotic triplets and quadruplets (Dionne Quintuplets being the only known living identical quintuplets), or a combination of monozygotes with dizygotes. Most common is a multiple birth which arises from multiple eggs being fertilized simultaneously.

>No idenitcal twins are in the same sac! Explain simese twins? They are
>joined together, idenical twins whose egg never divided all the way! SAME
>SAC!
>
>:o)
>
>Jennifer

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