Re: This is going to sound like a dumb question, but how does conception actually occur? I don't understand why doctors start counting the number of weeks pregnant from the last period. Doesn't ovulation happen after that?
You're right -- ovulation occurs about two weeks after the start of your period (the first day of your menstrual cycle). If the stars align, that's when conception happens too. Typically, one of your ovaries releases an egg 12 to 16 days after your cycle begins. It then travels from the ovary through the fallopian tube toward the uterus. But it only lives about 24 hours, so it must meet up with a sperm during that time to make a baby. Each ejaculation contains 30 to 300 million sperm cells (which can live in a woman’s body for up to 72 hours), but it only takes one to fertilize the egg.
However, those millions of swimmers encounter some obstacles along the way. The vagina is an acidic environment that's tough on sperm. Throw in the long distance from the cervix, through uterus, to fallopian tube and it's a pretty rough journey. If the sperm and egg do meet, the sperm must burrow into the egg's outer coating. Once the genetic material starts to combine, you've got yourself an embryo. The embryo then travels back down into the uterus and implants itself in the wall, and congratulations -- you're pregnant!
The reason for the two bonus weeks of pregnancy? Your doctor can't be sure exactly when you ovulated, but she can be sure of the date your cycle started, so she counts from then.
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