Once a woman learns that she is indeed pregnant, the very next thing she would want to do is to mark out her due date. Here begins the confusion of whether she should count 9 months from the current date or from the date that conception many have happened.
To simplify things, a woman needs to look at her pregnancy as a forty week period, during which various developmental changes occur in the growing fetus and at the end of which a fully formed baby will be born. An average pregnancy is said...
...to last for forty weeks, however it is normal for the pregnancy to last anywhere between 38 weeks and 42 weeks.
A woman’s forty week period does not in fact start from the date of conception, as this is often difficult to be certain of. It will begin from the date of the first day of her last menstrual period. This actually means that pregnancy starts even before fertilization or even ovulation, for that matter. This is done as the exact date of ovulation is also rather difficult to pin-point and the exact date of conception is even more uncertain as fertilization can take place up to 24 hours and longer after a woman ovulates. The sperm can also remain alive for up to three days after sexual activity. As such the date of conception just cannot be ascertained. Since the only date that can be established is the first day of the last menstrual period or LMP, pregnancy begins from that day on, for the purpose of dating it. As a result, the baby is actually younger by two weeks, since women usually experience ovulation a couple of weeks after menstruation begins. This also means that the baby spends 38 weeks in gestation by the time the fortieth week is reached. This may seem blurry and confusing, but this is the way in which the weeks comprising pregnancy are actually counted and also the basis of pregnancy calendars and due date calculators.
Once this becomes clear, you can narrow down on your estimated due date, by counting forty weeks from the start of your last menstrual period or thirty eight weeks from the date of conception. If you have trouble doing this manually with a calendar and a calculator, there are several much simpler options available for expectant mothers. A host of websites feature pregnancy due date calendars in which you just need to type in your LMP and in a matter of a few seconds, you will have your estimated due date.