A fetus's heart does not begin development until the fifth week of pregnancy, according to Medline Plus. Along with the heart, the spinal cord, brain and intestinal tract begin to form during the same time.
Medline Plus states that between weeks six and seven, the baby's heart begins beating faintly with a regular rhythm. During this same time, according to the Mayo Clinic, the brain begins developing nerves and blood starts flowing through the main blood vessels. After the tenth week, the fetus has now moved into the fetal period, during which the fetus starts resembling a baby. Around week 20, the mother starts feeling movement in her lower abdomen, and during week 22, the baby's heartbeat can be detected with a stethoscope.