The great divide. While you've been adjusting to the excitement of being pregnant, the new life inside you has been busy. The fertilized egg is implanting itself into the side of your uterus, where it continues its rapid development. Once implanted, the egg divides into layers of cells and officially becomes an embryo. These cell layers will grow into specialized parts of your little one's body, such as the nervous system, skeleton, muscles, and organs.

Support system under way. The placenta, the disk-like organ that connects your circulation to the embryo's, begins to form and attaches to the uterine wall at the site of implantation. The umbilical cord comes out of one side of the placenta. The amniotic fluid, which will cushion your little one throughout the pregnancy, is already forming inside an encircling sac, called the membranes.

Measuring up. By the end of the week, the embryo measures around 0.04 inch, or 1 millimeter—about the size of a poppy seed.

Signs of life. A missed period may still be the only indication that you're pregnant. You might, however, experience some spotting as your fertilized egg implants into the uterus. This light bleeding is known as implantation spotting, and it's completely normal.

Testing, testing. A home pregnancy test should show a positive result now, thanks to the hormone hCG, which the brand-new placenta is releasing into your body. This same hormone is largely responsible for the queasiness, or morning sickness, that many women experience in the first trimester of pregnancy.

Most home pregnancy tests are accurate after your first missed period, but false negatives do happen. Many women say they tested negative for several weeks after they stopped menstruating, even though they just "knew" they were pregnant (and were right).

From the experts. Now that you're pregnant, you may wonder which over-the-counter cough, cold, and allergy medicines are safe. "You want to avoid decongestants, antihistamines, and remedies containing PPA [phenylpropanolamine]," says Elaine Zwelling, R.N., Ph.D. "Some cold remedies and cough medicines contain as much as 25 percent alcohol," she adds, and these should be avoided as well. Click here to read more from Dr. Zwelling on what you can take when you're feeling under the weather. And remember always to check with your health care provider before taking any medications, even over-the-counter ones.