Your Baby's Development
Four weeks into your pregnancy, your baby (now an embryo) consists of two layers of cells — the epiblast and the hypoblast. These eventually develop into all of your baby's organs and body parts.
Two other structures that develop now are the amnion and the yolk sac. The amnion, filled with amniotic fluid, will surround and protect the growing embryo. The yolk sac will produce blood and help to nourish the embryo until the placenta takes over that role.
The embryo continues to implant in your uterus, burying itself deep within the endometrium. Some women have slight cramping and spotting during this week while implantation happens. They might mistake this for a period, especially because this is around the time their monthly period was due.
Once implanted, the embryo starts to make a hormone called human chorionic gonadotropin (hCG), which helps to maintain the lining of the uterus. It also sends a signal to the ovary to stop releasing an egg each month, which stops your monthly periods.
hCG is the hormone that's measured in pregnancy tests. This week, a pregnancy test probably could detect your pregnancy. hCG also causes the symptoms of pregnancy, which might start now. Tiredness, tingling or aching breasts, or nausea also can happen with premenstrual syndrome (PMS). But by the end of this week, your period won't happen. Instead, your pregnancy is under way!