Your due date will be calculated from the first day of your last menstrual period. Conception occurs about two weeks from this day, and that's when you're truly considered pregnant. In just 40 short weeks, your baby will grow from the size of a tiny seed to the size of a plump watermelon.
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Is it official yet? Not quite—but be assured that your body's got those baby-making mechanics well under way. After fertilization, your ovaries start ramping up the production of progesterone, a hormone that prepares your uterus to host the newly fertilized egg, or zygote, that will live there for the next 38 weeks or so.
So far your baby doesn't exist, but this is the week you ovulate. Your ovary releases a ripened egg (ovum) into your fallopian tube, where it will patiently await the sperm that have survived the 6- to 8-inch trek through your cervix and uterus. While 75 to 900 million sperm embark on this journey, less than a thousand actually make it past your cervix—and only one lucky swimmer will have the honor of penetrating the egg at the moment of conception.
Right now, you're just dying to find out: Am I pregnant or not? Unfortunately, you'll need to stick it out a bit longer. About four days after your egg is fertilized, it begins producing a hormone called human chorionic gonadotropin (HCG), which pregnancy tests can detect in about another week—first in your blood (via a test at your doctor's office) and then in your urine (which an at-home screening would spot).
There's no ultrasound image of your baby-to-be for weeks 1 and 2. While your health care provider counts these two weeks toward your due date, you aren't really pregnant. Confused? Your pregnancy due date is calculated using the first day of your last menstrual period (LMP). Obviously you weren't pregnant at that time, but it's the best reference your health care provider has for estimating baby's arrival day (until you get an ultrasound, which may provide a more accurate due date).