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Because one egg was fertilized by one sperm, the genetic material in the two embryos is the same.
At ovulation, the mucus in the cervix becomes more fluid and more elastic, allowing sperm to enter the uterus rapidly.
During this period, the embryo is most vulnerable to the effects of drugs, radiation, and viruses.
Therefore, a pregnant woman should not be given any live-virus vaccinations or take any drugs during this period unless they are considered essential to protect her health (see Drug Use During Pregnancy).
If more than one egg is released and fertilized, the pregnancy involves more than one fetus, usually two (twins).
Because the genetic material in each egg and in each sperm is slightly different, each fertilized egg is different. Identical twins result when one fertilized egg separates into two embryos after it has begun to divide.