Before she actually starts growing, you'll set the stage. Last week an increase in the amount of estrogen and progesterone coursing through your bloodstream prompted your uterus to form a lush, blood-rich lining of tissue to support a potential fertilized egg. At the same time, in your ovaries, eggs were ripening in fluid-filled sacs called follicles. At the beginning of this week (often around day 14 of a 28-day cycle), you ovulate: One of your eggs erupts from its follicle and is swept away from your ovary and into a Fallopian tube. During the next 12 to 24 hours that egg will be fertilized if one of the 250 million sperm (on average) your mate ejaculates manages to swim all the way from your vagina through your cervix, up into your uterus to the Fallopian tube and penetrates the egg. Only about 400 sperm will survive the arduous ten-hour journey to the egg, and only one will succeed in burrowing through its outer membrane. (It takes about 20 minutes for the lucky winner to find his way in.)
Over the next ten to 30 hours, the sperm's nucleus will merge with the egg's as they combine their genetic material. If the sperm carries a Y chromosome, your baby will be a boy; if it's an X chromosome, you'll be welcoming a girl. During the three- to four-day trip from your Fallopian tube to your uterus, the fertilized egg (now called a zygote) will divide into 16 identical cells. Once it enters the uterus, the zygote is called a morula. A day or two later, it will begin burrowing into the lush lining of your uterus, continuing its amazing growth and transformation. By this time your developing baby is just a little ball of cells that's officially referred to by scientists as a blastocyst: It has an inner cell mass that will become the embryo itself, a fluid-filled cavity that will become the amniotic sac, and an outer cell mass that will become the placenta, the pancake-shaped organ that delivers life-sustaining oxygen and nutrients to your baby and carries away her waste products.
How your life's changing
Your last period started 12 to 16 days ago, so you're probably ovulating now or will be soon. Strange as it may seem, your practitioner calculates your due date (and your baby's gestational age) starting from the first day of your last menstrual period. Pregnancy lasts about 38 weeks from conception, but since it's often difficult to pinpoint exactly when egg and sperm merged, doctors and midwives simply count 40 weeks of pregnancy beginning with the onset of your last period. That's why you're already considered to be two weeks into your pregnancy when fertilization occurs.
A lot of things have to fall into place for conception to happen — in this case, timing really is everything. To boost your odds, aim to have sex sometime between 72 hours before you ovulate and 24 hours after. (You're trying to accommodate the lifespan of both sperm, which survive for up to 72 hours, and the egg, which lives no more than 24 hours after ovulation.) Now's not the time to hesitate: Make sure your mate clears his calendar and that the two of you have plenty of time together to make love.
Before you retreat to the bedroom, though, you might want to do some homework: Read up on detecting ovulation to help you pinpoint your window of opportunity. Get the scoop on which sexual positions may help you conceive faster and even how you might influence your baby's gender. Finally, to spare you and your partner unneeded anxiety, learn how long it usually takes to get pregnant — most couples need more than a few tries before they can announce that a baby's on the way.
Last but not least, if you're trying to get pregnant and you haven't already stopped drinking, smoking, and taking drugs — even over-the-counter ones — now's the time to do so, since you want your body to be in the best possible shape for baby making. (If you're taking any prescription drugs, check in with your health provider to find out whether you should continue taking them.) And don't forget to take a daily multivitamin that contains at least 400 micrograms of folic acid (ideally starting three months before you want to conceive) to reduce your baby's risk of birth defects.