The stages of human fetal development are fertilization, development of the blastocyst, development of the embryo and development of the fetus and placenta, according to Merck Manual. Pregnancies can also be divided into three 3-month periods called trimesters.
The beginning of a pregnancy actually occurs on the first day of a woman's last menstrual period. This period, known as the menstrual age, typically occurs two weeks prior to conception, as reported by Cleveland Clinic.
Fertilization occurs after sexual intercourse, when the sperm penetrates the egg, forming a zygote. The zygote enters the uterus in three to five days In the uterus, where it develops into a hollow ball of cells called a blastocyst. Following an additional two to three days, the blastocyst attaches to the lining of the uterus, typically near the top. This process, known as implantation, is completed nine to ten days following fertilization, according to Merck Manual.
The next stage in development is the embryo within the amniotic sac, notes Merck Manual. During this stage, most internal organs and external body structures are formed. This stage typically occurs three weeks after fertilization, or five weeks into pregnancy.
The embryo is considered a fetus following the eighth week after fertilization, or 10 weeks after the beginning of pregnancy. By 12 weeks of pregnancy, the fetus fills the entire uterus, and by about 14 weeks, the sex can be identified. Following 24 weeks of pregnancy, the fetus has a chance to survive outside the uterus, as reported by Merck Manual.