Weeks 1 to 12
Week 1 This is the week of your last menstrual period before pregnancy. You won't conceive for another two weeks, but the first day of significant bleeding is considered the official start date of pregnancy perhaps because it's an easier date to recognize than the date of conception.
Week 2 In anticipation of new life, the uterus forms a blood-rich lining of tissue called the endometrium. At the same time, in one of the two ovaries, eggs ripen in fluid-filled sacs called follicles.
Week 3 Around midcycle (day 14 of the typical 28-day cycle), one of the eggs is swept into a fallopian tube. This is ovulation. If in the next 24 hours one of the 350 million sperm in the average ejaculate can trek all the way from the vagina through the uterus and to the fallopian tube to penetrate the egg, bingo -- fertilization. Sperm can live in the tube awaiting a mature egg for one to five days, so the lovemaking that created your baby could have taken place before ovulation. The fertilized egg immediately closes its outer membrane to other sperm and begins dividing into a cluster of identical cells as it floats down the fallopian tube to the uterus. If you are going to have twins, either one fertilized egg will split, creating identical twins, or two eggs will be separately fertilized, creating fraternal twins.
Week 4 The fluid-filled cluster nests inside the uterus, where it divides in two. The half attached to the uterine wall becomes the placenta; the other half will become the fetus. By the end of this week you'll miss a period, though you may experience staining called implantation bleeding.
Week 5 The ball of cells, about the size of an apple seed, has become an embryo. The placenta and umbilical cord, through which the baby will receive nourishment and oxygen, are on the job. This is when many women first suspect pregnancy; a home test can often confirm it. Schedule a visit to your doctor -- most first prenatal checkups take place between six and ten weeks -- and ask about taking a folate supplement, even before the appointment: Weeks five through ten are critical to neural development.
Week 6 The embryo looks more like a tadpole than a human. Its heart, no bigger than a poppy seed, has started beating. Major organs are developing, and the neural tube, which connects the brain and spinal cord, closes. You may begin to experience the physical sensations of pregnancy -- nausea, breast tenderness, fatigue, frequent urination. At your first prenatal visit, your doctor should test you for rubella immunity, Rh factor, and sexually transmitted diseases.
Week 7 The embryo is the size of a raspberry. Its head is large in proportion to the body. Dark spots where the eyes and nostrils will be, pits that mark the ears, and protruding buds that will become the arms and legs are beginning to form.
Week 8 The embryo has distinct slightly webbed fingers and toes and see-through-thin skin. Your uterus is now the size of a small orange.
Week 9 The strawberry-size being is now a "fetus" and is constantly moving, though you won't feel it yet. What you probably will notice is that you're spilling out of your old bra and need better support.
Week 10 In both shape and size, the fetus resembles a medium shrimp. Already, its genitals are beginning to form, though your doctor can't yet tell the sex by looking at a sonogram.
Week 11 The fetus, about two inches long and less than half an ounce, is swallowing and kicking. Its vital organs are in place and each day more minute details fill in. The rapid "whoosh" of its heartbeat can be heard through a special stethoscope. Your uterus is the size of a grapefruit.
Week 12 Most likely, your nausea begins to wane now, and energy picks up. The uterus moves from the pelvic floor to front-and-center in your abdomen, relieving pressure on your bladder. Now about two and a half inches long, the fetus is fully formed, from tooth buds to toenails. With the most critical development past, your chances of carrying and delivering a healthy baby greatly increase.