Pregnancy lasts an average of 280 days (40 weeks) from the first day of your last menstrual period (LMP). The first day of your LMP is considered day one of pregnancy, even though you probably didn’t conceive until about two weeks later (fetal development lags two weeks behind your pregnancy dates).
Calculating your due date isn’t an exact science. Very few women actually deliver on their due date, so, while it’s important to have an idea of when your baby will be born, try not to get too attached to the exact date.
If you have regular 28-day menstrual cycles, there are two ways to calculate your due date.
Naegele’s rule involves a simple calculation: Add seven days to the first day of your LMP and then subtract three months.
For example, if your LMP was November 1, 2017:
- Add seven days (November 8, 2017).
- Subtract three months (August 8, 2017).
- Change the year, if necessary (to the year 2018, in this case).
In this example, the due date would be August 8, 2018.
The other way to calculate your due date is to use a pregnancy wheel. This is the method that most doctors use. It’s very easy to estimate your due date if you have access to a pregnancy wheel.
The first step is locating the date of your LMP on the wheel. When you line up that date with the indicator, the wheel displays your due date.
Remember that the due date is only an estimate of when you will deliver your baby. The chances of actually having your baby on that exact date are very slim.
This is more common than you’d think. Luckily, there are ways to figure out your due date when you can’t remember the first day of your LMP:
- If you know you had your LMP during a particular week, your doctor can estimate your due date accordingly.
- If you have no idea when your last period was, your doctor may order an ultrasound to determine your due date.
Some women have cycles that are consistently longer than the average 28-day cycle. In these cases, a pregnancy wheel can still be used, but some simple calculations are necessary.
The second half of a woman’s menstrual cycle always lasts for 14 days. This is the time from ovulation to the next menstrual period. If your cycle is 35 days long, for example, then you probably ovulated on day 21.
Once you have a general idea of when you ovulated, you can use an adjusted LMP to find your due date with a pregnancy wheel.
For example, if your menstrual cycle is usually 35 days long and the first day of your LMP was November 1:
- Add 21 days (November 22).
- Subtract 14 days to find your adjusted LMP date (November 8).
After you calculate your adjusted LMP date, simply mark it on the pregnancy wheel and then look at the date where the line crosses. That is your estimated due date.
Some pregnancy wheels may allow you to enter the date of conception — which occurs within 72 hours of ovulation — instead of the date of your LMP.
Your doctor may change your due date if your fetus is significantly smaller or larger than the average fetus at your particular stage of pregnancy.
Generally, your doctor orders an ultrasound to determine the gestational age of your baby when there’s a history of irregular periods, when the date of your LMP is uncertain, or when conception occurred despite oral contraceptive use.
An ultrasound allows your doctor to measure the crown-rump length (CRL) — the length of the fetus from one end to the other.
During the first trimester, this measurement provides the most accurate estimation for the age of the baby. Your doctor may change your due date based on the ultrasound measurement.
This is most likely to occur in the first trimester, especially if the date estimated by the ultrasound differs by more than one week from the date estimated by your doctor based on your LMP.
In the second trimester, an ultrasound is less accurate and your doctor probably won’t adjust your date unless the estimates vary by more than two weeks.
The third trimester is the least accurate time to date a pregnancy. Estimates based on an ultrasound can be off by as much as three weeks, so doctors rarely adjust dates during the third trimester.
However, it’s not uncommon for a doctor to perform an ultrasound in the third trimester if they’re thinking about changing your date.
A repeat ultrasound provides valuable information about the growth of the fetus and may reassure you and your doctor that the change in due date is reasonable.
Did you know?
Ultrasound measurements for estimating the age of a fetus are more accurate during the early stages of pregnancy. In the first few weeks, fetuses tend to develop at the same rate. However, as pregnancy progresses, the rates of fetal growth begin to vary from pregnancy to pregnancy.
This is why ultrasound measurements can’t be used to accurately predict the age of the baby in the later stages of pregnancy.
Ultrasounds are not a necessary part of prenatal care. Avoid numerous scans and have ultrasounds for medical reasons only.
When a doctor performs an ultrasound, they write a report on the findings and include two estimated due dates. The first date is calculated using the date of the LMP. The second date is based on the ultrasound measurements. These dates are rarely the same.
When your doctor evaluates the ultrasound results, they’ll determine whether or not these dates are in agreement. Your doctor probably won’t change your due date unless its significantly different from your ultrasound date.
If you have more ultrasounds, each ultrasound report will contain a new due date based on the most recent measurements. An expected due date shouldn’t be changed based on measurements from a second- or third-trimester ultrasound.
Due date estimations are more accurate earlier in pregnancy. Later ultrasounds are helpful in determining whether the fetus is growing well but not for determining the age of the fetus.