The pregnancy lasts about 280 days (40 weeks), starting on the first day of the last menstrual period. The first day of the last menstrual period is considered the first day of pregnancy even though most pregnancies are not conceived until two weeks later (the development of the fetus is usually two weeks behind the date of conception). Knowing your due date is important as it helps you plan and prepare for your baby’s arrival. Moreover, it gives your provider all the information they need to keep an eye on your health and your baby's health as well. In this article, we will learn how to estimate the date of birth in various ways.
How Many Weeks Pregnant Am I
There are three trimesters of pregnancy, each marked by specific developmental stages of the fetus.
How are due dates usually calculated?
Gestational age is measured from the first day of the last menstrual period (LMP).
If the woman has a regular period which lasts 28 days, there are two ways to calculate fetal age:
Nagel's rule for calculating the due date
Naegle's rule involves a simple equation: we add 7 days to the date of the first day of the last menstrual period, and then subtract three months, to get the estimated date of delivery. For example, if your last menstrual period was November 1, 2021:
- Add 7 days to the date, so it becomes November 8, 2021.
- Subtract three months, so the date becomes August 8, 2022.
- Change the date when necessary, so in our example the date is 2022 instead of 2021
Pregnancy wheel for calculating Due Date
Another way to calculate pregnancy and the expected delivery date is to use the pregnancy wheel, which is used by most doctors to know the due date.
Put the date of your last menstrual period on the wheel, and it will display your expected delivery date.
Please keep in mind that the chances of giving birth on that exact date are usually small, but around that time these methods can only give you an estimate.
How to calculate the due date if you do not remember the date of the last menstrual period?
This condition is more common than you think. But fortunately, by using ultrasound, you can determine the gestational age and the due date.
What is the solution in an irregular or somewhat long period?
There are some women who have menstrual periods that are consistently longer than normal. With some simple calculations, the pregnancy wheel can still be used in these cases. The second phase of the menstrual period always lasts 14 days, which is the time between ovulation and the next menstrual period. If a woman's menstrual period is 35 days, for example, then the ovulation will be around day 21 of her period. As long as we have a general idea of when ovulation will occur in general, the adjusted LMP can be used to determine the date of delivery on the gestational wheel. For example, if the menstrual period is 35 days long, and the first day of the period is November 1st (1/11):
- Add 21 days (the date becomes November 22).
- Subtract 14 days to find the adjusted date of the last menstrual period (here, November 8).
- Then we can find the estimated date of birth on the gestational wheel as before.
Can my due date change? Why might the doctor change the expected due date?
Don't worry, your doctor may change the expected delivery date if the fetus is clearly smaller or larger than average depending on the gestational age. There are a number of reasons as your pregnancy progresses:
- You have irregular periods.
- Your first date relied on Doppler, not an ultrasound.
- Your first ultrasound was in the second trimester.
Did you have your first ultrasound in your second trimester? If so, this could be another reason for the shift in your due date. According to the American College of Obstetrics and Gynecology (ACOG), ultrasounds performed in the first trimester give the most accurate due date predictions.
- Maybe your uterus size doesn’t sync up with the standard growth charts (for example, you measure big).
In fact, only one in every twenty people delivers on their estimated due date, implying that only 5% of babies are born on time. So, your due date is just an estimate; labor is likely to begin two weeks before or after this date. Pregnancies generally last between 38 and 42 weeks (or 280 days). Babies born before 37 weeks are considered premature.