How to Calculate the Date in Pregnancy?

Pregnancy lasts an average of 280 days (40 weeks) from the first day of your last menstrual period (LMP). Although you probably won’t conceive until two weeks later, the first day of your last menstrual period is considered the first day of pregnancy (fetal development is two weeks behind your pregnancy dates).

Calculating your due date is not an exact science. Few women give birth by due date, so while it’s important to have an idea of ​​when your baby is due, try not to get too hung up on the exact date.

How Can I Calculate the Due Date of Pregnancy?

How To Calculate Pregnancy By Months, Weeks & Trimesters

If you have regular 28-day menstrual cycles, there are two ways to calculate your due date.

Naegele’s rule

The Naegele rule involves a simple calculation: Add seven days to the first day of your last menstrual period and subtract three months.

For example, if your last menstrual period was November 1, 2017:

  1. Add seven days (November 8, 2017).
  2. Subtract three months (August 8, 2017).
  3. Change the year if necessary (in this case to 2018).

In this example, your pregnancy due date is August 8, 2018.

pregnancy wheel

Another way to calculate your due date is to use a pregnancy wheel. This is the method most doctors use. If you have access to a pregnancy wheel, it is very easy to guess the due date.

Ovulation Calendar & Pregnancy Wheel — Medshop Australia

The first step is to find the date of your last menstrual period on the wheel. When you align this date with the indicator, the wheel displays your due date.

Remember that the due date is only an estimate of when you will meet your baby. The chances of having your baby exactly on this date are very low.

What Happens If You Don’t Know the Date of Your Last Menstrual Period?

This is more common than you might think. Fortunately, there are ways to tell your due date when you can’t remember the first day of your last period:

  • If you know you have your last menstrual period during a certain week, your doctor can estimate your due date accordingly.
  • If you have no idea when your last period is, your doctor may order an ultrasound to set your due date.

What Happens If You Have Irregular or Long Cycles?

Some women have consistently longer cycles than the average 28-day cycle. In these cases, a pregnancy wheel can still be used, but some simple calculations are required.

The second half of a woman’s menstrual cycle always lasts 14 days. This is the time from ovulation to the next menstrual period. For example, if your cycle lasts 35 days, you will likely ovulate on day 21.

Once you have a general idea of ​​when you ovulated, you can use a set last menstrual period to find your due date with the pregnancy wheel.

For example, if your menstrual cycle is usually 35 days long and the first day of your last period is November 1st:

  1. Add 21 days (November 22).
  2. Subtract 14 days (November 8th) to find your last adjusted period date.

After calculating the last menstrual period date you set, mark it on the pregnancy wheel and look for the date the line intersects. This 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.

What Does It Mean If Your Doctor Changes the Date of Birth?

If your fetus is significantly smaller or larger than the average fetus at a certain stage of your pregnancy, your doctor may change your due date.

In general, your doctor will give you an ultrasound to determine your baby’s size and condition when there is a history of irregular periods, your last menstrual period is uncertain, or when you are pregnant despite the use of oral contraceptives.

An ultrasound allows to measure the length of the fetus from one end to the other.

During the first three months, this measurement provides the most accurate estimate of the baby’s age. Your doctor may change your due date based on the ultrasound date.

This is most likely to occur in the first trimester, especially if the date predicted by the ultrasound differs by more than a week from the date predicted by your doctor based on your last menstrual period.

In the second trimester, ultrasound is less accurate and your doctor probably won’t be able to adjust the date unless estimates change by more than two weeks.

The third trimester is the least accurate time to date a pregnancy. An ultrasound-based estimate can be up to three weeks off, so doctors rarely adjust dates in the third trimester.

However, it’s not uncommon for your doctor to do a third trimester ultrasound if they’re considering changing your date.

A repeat ultrasound provides valuable information about the growth of the fetus and can assure you and your doctor that the change in due date is reasonable.


Ultrasound measurements are more accurate in the early stages of pregnancy to estimate the age of a fetus. In the first few weeks, fetuses tend to develop at the same rate. However, as pregnancy progresses, fetal growth rates begin to change from pregnancy to pregnancy.

Therefore, ultrasound measurements cannot be used to accurately estimate the baby’s age in the later stages of pregnancy.

Ultrasounds are not a necessary part of prenatal care.

Why Does the Ultrasound Date Differ from the Due Date?

When a doctor does an ultrasound, she writes a report on the findings and includes two estimated due dates. The first date is calculated using the date of the last menstrual period. The second date is based on ultrasound measurements. These dates are rarely the same.

When your doctor evaluates the ultrasound results, he or she will determine if these dates are compatible. Your doctor probably won’t change your due date unless it’s significantly different from your ultrasound date.

If you get more ultrasounds, each ultrasound report will include a new deadline based on the most recent measurements. An expected deadline should not be changed based on measurements from a second or third trimester ultrasound.

Due date estimates are more accurate in early pregnancy. Subsequent ultrasounds are helpful in determining whether the fetus is growing well, but not in determining the age of the fetus.

Healthline, How to Calculate Your Due Date, 2018


  • ACOG reinvents the pregnancy wheel: Launches new due date app. (2016).
  • Avoid fetal “keepsake” images, heartbeat monitors. (2014).
  • Calculating a due date. (n.d.).
  • Calculating your estimated due date. (2014).
  • Due date calculator. (n.d.).
  • Pregnancy due date and gestational age calculator. (n.d.).

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