The three stages of prenatal development are the germinal stage, the embryonic stage and the fetal stage. These stages begin two weeks after conception.
Prenatal development takes about 38 weeks to complete and goes through these three developmental stages.
The first stage is the germinal stage. The germinal stage occurs during the first two weeks of a baby's development. During this time, the zygote moves from the Fallopian tube toward the uterus and rapidly divides. When it reaches the uterus, the zygote implants itself on the uterine wall and the germinal stage ends when the zygote is firmly implanted.
The embryonic period lasts from implantation until around the 10th week of pregnancy. During this time, all important structures, such as the placenta, are formed and this is generally considered the most important time in a baby's development. Once the embryonic period has ended, the baby is considered a fetus.
The fetal period takes place from the 10th week of pregnancy and lasts until the baby is born. This is the time when the baby grows very quickly. An embryo weighs around 1/10th of an ounce and is roughly 1 1/2 inches long during the embryonic period and, during the fetal period, may grow to be more than 20 inches long and weigh more than 7 pounds.