By the time she has been pregnant for 4 weeks, a woman may just be suspecting that she is pregnant. Deep inside her womb, however, much has been going on. The growing egg, called an embryo at this point in the pregnancy, doesn’t look anything like a baby, but it is developing microscopic internal organs and systems that clearly identify it as a human.
Implantation and Division
By the time of the fourth week of pregnancy, the fertilized egg has traveled down the fallopian tube and has implanted deeply into the uterine wall. At this point, a cavity has formed at the center of the blastocyst or cluster of cells. Cells on one side will become the embryo, and the other side will become the placenta. The amniotic fluid is forming, as is the yolk sac. This yolk sac will nourish the embryo during the early days of pregnancy.
Size and Appearance
By 4 weeks, the fertilized egg looks like a tiny tadpole. Although it has grown 10,000 times larger than when it was first conceived, it is approximately 1mm long and weighs less than a single gram—about the size of a sesame seed.
The tiny embryo is beginning to form a head and a body. On the head, there is an opening for the mouth and two folds on either side that will become the ears. Further down on the body are little buds that will become the arms and legs.
The embryo has three sections that will eventually become the different parts of your infant’s body. The inner layer, known as the endoderm, will form into the intestines, liver and lungs. The middle layer, known as the mesoderm, will form into the bones, muscles, heart and reproductive system. The outer layer, known as the ectoderm, will form the hair, eyes, skin and central nervous system.
By the fourth week of pregnancy, the blood is beginning to be pumped through the tiny embryonic heart and circulatory system. If a mother were to have an ultrasound done at this time, the embryo’s beating heart would be visible.