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Stem cells are undifferentiated cells that are capable of dividing for long periods of time and can give rise to specialized cells under particular conditions. Embryonic stem cells are a particular type of stem cell derived from embryos. According to US National Institutes of Health (NIH), in humans, the term "embryo" applies to a fertilized egg from the beginning of division up to the end of the eighth week of gestation, when the embryo becomes a fetus. Between fertilization and the eighth week of gestation, the embryo undergoes multiple cell divisions.
In humans, multi-fetal pregnancy occurs when a mother carries more than one fetus during the pregnancy. The most common multi-fetal pregnancy is twins, but mothers have given birth to up to eight children (octuplets) from a single pregnancy. Multiple fetusus can result from the release of multiple eggs or multiple ovulations, the splitting of a single fertilized egg, and fertility treatments such as in vitro fertilization (IVF) which involves the insertion of many fertilized eggs into the mother's uterus.