During the fifth week of fetal development, a baby begins to develop its heart, muscles, bones, spinal cord and brain, according to WebMD. During this time, the umbilical cord connects to the baby to supply blood, oxygen and nutrition. The placenta and amniotic fluid begin to form in the womb.
Parents magazine explains the heart starts to divide into four chambers, and tiny cardiac muscle cells begin to contract. The yolk sac disappears as the umbilical cord attaches to the uterus. At five weeks, the embryo is the size of an apple seed, and it looks like a tadpole with a tail. Pregnancy symptoms such as morning sickness, tender breasts, fatigue and more frequent urination may appear at this stage.
Baby Center states that three distinct layers begin to separate into various organs. The top layer, or ectoderm, contains the neural tube that forms the spinal cord, brain, backbone and nerves. The middle layer, or mesoderm, contains the heart that beings to pump blood at five weeks. The endoderm, or innermost layer of tissue, forms other internal organs later in the pregnancy.
During the sixth week of fetal development, limbs start to form, and the eyes begin to distinguish themselves on the surface. When a doctor does an ultrasound, a heartbeat may be heard at this time, according to WebMD.