Twin

The term twin most notably refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and are usually, but not necessarily, born on the same day. A fetus alone in the womb is called a singleton. Due to the limited size of the mother's womb, multiple pregnancy is much less likely to carry to full term than singleton birth (twins usually around 34 to 36 weeks). Since some premature births often have health consequence to the babies, twins birth are more often handled with special procedures than regular births.

Complications of twin pregnancy

Vanishing twins

Main article: Vanishing twin

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Researchers suspect that more pregnancies start out as multiples than come to term that way. Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes.

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Conjoined twins

Main article: Conjoined twin

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Conjoined twins are monozygotic twins whose bodies are joined together at birth. This occurs where the single zygote of identical twins fails to separate completely. This condition occurs in about 1 in 100,000 pregnancies.

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Parasitic twins

Main article: Parasitic twin

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Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a parasite towards the other.

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Miscarried twin

Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. Similar to vanishing twin.

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