Congratulations! You have just discovered that you’re pregnant. Now you may be wondering how far along in your pregnancy you are, and when the baby is due to be born.
Most gestations (the time that you carry the baby) are figured at 40 weeks from the first day of your last menstrual period. Your menstrual period is a useful marker since it is a monthly event; however, ovulation (release of an egg) and subsequent fertilization can occur anywhere from 7-21 days after your menstrual period. In general, ovulation is estimated to be day 14 which means the actual duration of pregnancy is 38 weeks. Here are several ways to determine your due date:
FROM YOUR LAST MENSTRUAL PERIOD (LMP)
If you know when the first day of your LMP was, subtract 3 months and add 1 week. So if your LMP was December 1, your due date would be September 8th the following year.
If you have a pregnancy wheel, line up the “first day of last period” arrow with the date of your LMP. Your “probable day of delivery” arrow (40 weeks) will line up with your due date.
FROM THE DATE OF CONCEPTION
If you actually know the date you conceived, add 38 weeks (or subtract 3 months + 1 week). So if your conception date was December 1, your due date would be August 24th of the following year.
IF YOU DON’T KNOW WHEN YOUR LMP WAS
If you have irregular menses, it can be difficult to know how far along you are, as it is not usually known when ovulation occurs. An ultrasound in the first trimester is accurate within a few days. Your health professional may want to schedule you an extra 4 weeks out from your positive pregnancy test to make sure you are far enough along (at least 8 weeks from LMP) so that a heartbeat can be found and the baby can be measured.
FROM AN ULTRASOUND
In the first trimester (8-13 weeks) an ultrasound due date is quite accurate, within 3-4 days. After 28 weeks an ultrasound due date can be off by as much as 2-3 weeks. Because of that, your health professional will usually use an earlier ultrasound as the predictor of your due date.
HOW DO I KNOW WHEN I OVULATE?
Although you don’t actually know when you ovulate, in a 28 day cycle it usually occurs around 14 days after the first day of your menstrual period. There is a six-day “fertile” window; the five days leading up to ovulation, plus the ovulation day itself. There are test kits on the market which measure the luteinizing hormone (LH) detected in your urine. Ovulation typically occurs about 10 to 12 hours after LH peaks. There are also fertility monitors, which measure both the LH and estrogen levels, to give you the five most fertile days. Of course, the fertility monitors are quite a bit pricier than the ovulation predictor kits.
There are several phone apps that are useful in determining an estimated due date and gestational age but keep in mind that a 1st trimester ultrasound is the most accurate way to determine your due date. Therefore, establishing care with a health provider early in pregnancy is recommended.