What's going on in your womb now? A lot. Your baby-in-the-making is just a tiny ball consisting of several hundred cells that are multiplying madly. Once the ball of cells (called a blastocyst) takes up residence in your uterus, the part of it that will develop into the placenta starts producing the pregnancy hormone hCG (human chorionic gonadotropin), which tells your ovaries to stop releasing eggs and triggers increased production of estrogen and progesterone (which keep your uterus from shedding its lining — and its tiny passenger — and stimulates placental growth). HCG is the hormone that turns a pregnancy test positive; by the end of this week, you may be able to take one and get a positive result! (If your test is negative and you still haven't gotten your period in two or three days, try again then.)
Meanwhile, amniotic fluid is beginning to collect around your ball of cells in the cavity that will become the amniotic sac. This fluid will cushion your baby in the weeks and months ahead. Right now, your little blastocyst is receiving oxygen and nutrients (and discarding waste products) through a primitive circulation system made up of microscopic tunnels that connect your developing baby to the blood vessels in your uterine wall. The placenta won't be developed enough to take over this task until the end of next week.
How your life's changing:
A momentous meeting has taken place inside you — a single sperm has broken through the tough outer membrane of your egg and fertilized it. Several days after conception, the fertilized egg burrowed into the lining of your uterus and started to grow. A baby is in the making! You probably don't know you're pregnant yet, but you may notice a little spotting by the end of this week. This so-called "implantation spotting" may be caused by the egg burrowing into the blood-rich uterine lining (a process that began last week at 6 days after fertilization), but no one knows for sure. In any case, the spotting is very light and only a minority of pregnant women experiences it at all.