How the Length of Pregnancy is Calculated
When we think of pregnancy and when it actually begins, most of us automatically assume that it starts when egg and sperm join to create a new life. This is technically true. A woman is “with child” (an old-fashioned way of saying pregnant) from the moment of conception.
Medical professions don’t think of pregnancy that way. Midwives, obstetricians, and other healthcare providers calculate the length of pregnancy using the date of your last menstrual period (which is the first day you started bleeding.) In medical circles, using the estimated date of conception is too imprecise. Most women can’t pinpoint the exact day they got pregnant, but they can tell doctors the date that their last period started.
So, the first day of your period is considered day 1 of pregnancy. (Even though you aren’t technically “with child” or pregnant yet.)
This may be confusing, but it makes perfect sense to doctors. Knowing when your last menstrual cycle was helps doctors estimate roughly the date of conception.
The average woman’s menstrual cycle lasts 28 days. In a nutshell, there will be 28 days from day 1 of your period to the first day of your next period. (Twenty-eight days is the average menstrual cycle, but the length of a menstrual cycle can vary from woman to woman. A normal cycle can range anywhere from 21 to 35 days.)
Most women will ovulate 14 days (or 2 weeks) after the start of their period. If you have sex during the time you ovulate, you have a high chance of conceiving. So the date of conception is estimated to be 2 weeks after your last menstrual period.
You are technically pregnant when you’re considered “2 weeks pregnant.” When you are “4 weeks pregnant,” your future child has only been in your womb for two weeks.
Why don’t doctors use estimated ovulation date?
Using the date of your last menstrual period is more precise that using your estimated ovulation date. There are a number of factors that can change ovulation, including stress, illness, or normal routine disruption. Any of these factors can push up ovulation – make it occur sooner in the month, or occur later than expected.
Ovulation is an internal process – you can’t actually see your egg being released from the ovaries, and having your period is external. There is no way that you can miss having your period. (Unless you are blind, or you can’t feel the icky, wet mess.)
Length of Pregnancy
Now that you understand why the length of pregnancy starts on the last day of your menstrual period (LMP for short), you should understand why there is 280- day rule.
A human pregnancy lasts 280 days (or roughly 40 weeks). But remember, day 1 of your pregnancy is calculated as the last day of your period. If you give birth when you are 40 weeks pregnant, you were technically only pregnant for 266 days.
* The length of an actual pregnancy is 266 days – from the moment of conception to childbirth. *
If you have an abnormal menstrual cycle (either longer or shorter than the average 28 day cycle), you will want to tell your doctor this. This may change your “estimated due date.” Talk to your doctor about any of your concerns.
Easy Formula to Calculate Your Estimated Due Date
Just for fun, now that you understand how the length of pregnancy is calculated, you can calculate your baby’s estimated due date, if you know the date of your last menstrual period.
You can use Naegele’s Rule to predict your estimated due date. Developed by a German obstetrician, Naegele’s Rule assumes that you have a 28 day menstrual cycle and ovulation occurring on day 14. It does not take into account leap years or months with fewer than 31 days.
Here’s how to calculate your estimated due date:
- Take the first day of your last period
- Add seven days to your LMP
- Subtract three months
- Add one year.
For example, if my last menstrual period were on January 11, 2011, my due date would be October 18, 2011.