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In the past, doctors used the first day of the mother's last menstrual period to calculate due dates. This was because the exact date of conception was hardly ever known for sure. These days, with pregnancy tests that can confirm pregnancy only a couple of days after conception, it might be easier to make an accurate guess. Keep in mind that few babies ever come right on schedule.
To calculate your due date from conception, use these simple rules:
This makes sense because conception occurs when women ovulate, and this happens in the middle of a monthly menstrual cycle. Subtracting two weeks if you use the date of conception from the calculation based on the first day of a period is a good rule of thumb.
For example, you might have had your last period on Valentine's Day. That means you probably conceived right around the first of March. You should be prepared to have your baby in time for Thanksgiving.
What if you do not know your conception date or the first day of your last period?
Women have been known to lose track of these things, but an ultrasound as soon as five or six weeks after conception can usually help estimate the baby's gestational age very accurately.
Determining your conception date can be simple or complicated depending on whether your pregnancy was planned or a surprise.
If you are planning your pregnancy, you are likely either actively "trying" which might include tracking your ovulation patterns and keeping track of when you have intercourse.
It usually takes 4-6 weeks for a pregnancy to show up on an at home pregnancy test, this is because the HCG levels that are present during pregnancy begin multiplying at a faster rate around this time.
Your doctor will give you a due date based on your last period, this actually means they consider you pregnant around two weeks before you even conceive. Physicians consider a pregnancy 280 days long, beginning from the date of your last menstrual period. One way to figure out the date of conception is to take your due date and subtract 266 days. This takes the 14 days from the beginning of your last menstruation and has you calculate from the average day of ovulation.
If you have been charting your ovulation patterns you will probably easily be able to figure out your date of conception. In this case all you would do is look on your calendar to see when you ovulated and when you had intercourse. Conception will always take place within within twenty-four hours of ovulation. The day you had intercourse is not necessarily your conception date. Sperm can live in your body for as long as five days, so if you have intercourse on Monday, and ovulate on Thursday, fertilization could occur making Thursday your conception date.
If you don't like math, there are plenty of online calculators that you can use to do the subtracting for you, just input the dates the calculator asks for and you'll get your date.