A question for 9 out of 10 first-time pregnant women, figuring out how to calculate pregnancy duration can be very confusing. The calculation doctors make seems like a big mystery! But know that it’s not that complicated to understand how doctors do the pregnancy duration calculation. You just need to keep in mind that pregnancy lasts on average 280 days, that is, 9 months distributed over 40 weeks or from 37 to 42. Actually, it all depends on your cycle and when your fertile period was.
Essential Information
A woman with a regular menstrual cycle wouldn’t have major problems calculating her pregnancy duration, but another woman with an irregular cycle or who doesn’t remember when her period was could have trouble doing the calculation. The fact is that the most important factor to determine pregnancy duration is to use your last menstrual period as a reference.
Initially, the doctor will consider the first day of menstrual bleeding even if the woman has an irregular cycle. He or she will count from the first day of the menstrual cycle because there’s no certainty about when conception actually took place. This makes the calculation a little less uncertain.
Counting by Months
In our mothers’ time, pregnancy was counted by months. It’s not uncommon to see an older woman confused when we talk about pregnancy weeks, since this calculation is relatively new. About 18 years ago there was a change in how pregnancy is counted, as this gives a real idea of when the months are completed.
Did you know that some months of pregnancy can be longer than others? Yes! The last months of pregnancy have about 5 weeks each. Since some months are longer, the correct way is to count by weeks so neither the doctor nor the mom gets confused calculating pregnancy duration.
Counting Calculation
But how can you understand the calculation if pregnancy is counted from the first menstrual day, when you’re not even pregnant yet? Simple: the doctor considers 38 weeks within this calculation. Normally a cycle is about 28 or 35 days. The doctor subtracts 14 days or 2 weeks from the total pregnancy weeks: 40-2 = 38.
These 38 weeks can be considered the real duration of a pregnancy and can extend up to a true 40 weeks, that is, 42 weeks of pregnancy. We can say that a woman with longer cycles might take a few more days to complete the 280 days of pregnancy and go into labor.
That’s why it’s important to remember that the baby can be born up to 42 weeks counting from the first day of menstruation, and everything depends on when conception happened. To understand better, let’s look at a simple calculation:
38 weeks are about 266 days of pregnancy
9 months are equivalent to 270 days of pregnancy
So there’s just a 4-day difference between the two, which is negligible in terms of pregnancy duration. Ideally, you should have at least 266 days of pregnancy, up to 280 days if you consider 42 weeks. We can say that a woman who got her period on July 31 will have an estimated due date 280 days later, which is March 8.
Doctors have a chart that makes this calculation much easier. To make it a little more (or less) confusing, I’ve found a more exact and close way doctors do the calculation: A) Determine the first day of your last period B) add 7 days C) subtract 3 months D) add 12 months. For example:
LMP July 31, 2011 + 7 = August 7
August 7 – 3 months = May 7, 2011
May 7, 2011 + 12 months = May 12, 2012 Estimated due date (EDD)
Ways to Calculate Pregnancy Duration
In addition to the medical calculation, a very effective way that can make all the difference in how to calculate pregnancy duration is the ultrasound. It will show, through the size of the baby or the gestational sac, which stage the pregnancy is at based on the measurements taken by the device. This is the most reliable way to know exactly how far along you are, and, in fact, the first ultrasound will be taken into account by the doctor until the end of the pregnancy.
Video about Pregnancy Weeks
Weeks at the End of Each Month
One of the most common problems for pregnant women is knowing when a pregnancy week closes. When, exactly, the week changes from one to the next. You should mark the change when the week moves, for example: from 4.6 to 5. If that day falls on a Friday, then you will count the week change every Friday. To help with the months, here is a little chart to make it easier to understand the end of each month:
| 4 + 3 | End of 1st month |
| 8 + 5 | End of 2nd month |
| 13 + 1 | End of 3rd month |
| 17 + 4 | End of 4th month |
| 21 + 6 | End of 5th month |
| 26 + 2 | End of 6th month |
| 30 + 4 | End of 7th month |
| 35 + 0 | End of 8th month |
| 40 + 0 | End of 9th month |
At the end of pregnancy, the initial calculation is essential since the baby’s measurements become different and may make the calculation of pregnancy duration more difficult. That’s because if the baby is large there will be a positive difference and labor may happen earlier than expected.
If the baby is small it can confuse the doctor, making him or her think it’s less time than it is, so delivery may go overdue if labor does not start on its own. A useful tip is to keep your first ultrasound and always take it with you in case you need to go to the maternity ER—although your prenatal booklet contains all this information, better safe than sorry. Still have questions about how to calculate pregnancy duration? Visit the site’s calculator!
See also: Pregnancy Weeks – How to Convert to Months?
Photo: Hatty Photo
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Key Information Box:
- A question for 9 out of 10 first-time pregnant women, figuring out how to calculate pregnancy duration can be very confusing. The calculation doctors make seems like a big mystery!
- The fact is that the most important factor to determine pregnancy duration is to use your last period as a reference.
- In our mothers’ time, pregnancy was counted by months. It’s not uncommon to see older women confused when we talk about pregnancy weeks, since this calculation is relatively new.
- These 38 weeks can be considered the real duration of a pregnancy and can be extended up to a true 40 weeks, that is, 42 weeks of pregnancy.
- The ultrasound will show, through the size of the baby or the gestational sac, which stage the pregnancy is at based on the measurements taken by the device.
