Four weeks into your pregnancy, your baby (called an embryo) consists of two layers of cells — the epiblast and the hypoblast — that will eventually develop into all of your baby's organs and body parts. Two other structures that develop at this time 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.
This week your baby continues to implant in your uterus, burying itself deep within the endometrium. Once implanted, your baby starts to produce 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. Some women experience slight cramping and spotting of blood during this week while implantation is taking place, and they may mistake this for a period, as it often occurs around the time their monthly period was due.
hCG is the hormone that is measured in pregnancy tests. This week a pregnancy test will probably be able to detect your pregnancy! hCG also causes the symptoms of pregnancy, which can appear this week. Fatigue, tingling or aching breasts, or nausea might lead you to believe your period will be starting any day because the first pregnancy symptoms resemble premenstrual syndrome (PMS). But by the end of this week, your expected period will not take place. Your pregnancy is well on its way!
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