In humans, a woman becomes pregnant after an ovum and sperm unite (fertilization). After fertilization, a zygote forms. This zygote grows into a fetus after the eighth week of pregnancy. The fetus receives nutrition from the placenta. The umbilical cord connects the fetus and placenta. A sac-like structure, the amniotic sac, envelopes the fetus. The sac contains amniotic fluid, which cushions the fetus and protects it from harm.