Pregnancy EDD (Due Date Calculator) – Meaning, Formula, Types

Pregnancy EDD (Due Date Calculator) – Meaning, Formula, Types

29 Dec 2017 | 6 min Read


Author | 1369 Articles

Waiting for the baby’s birth is exciting as well as stressful for the parents-to-be. Getting an idea of the date of the baby’s arrival ahead of its birth helps them to prepare for pregnancy week by week. Doctors use several methods to calculate the due date or the estimated date of delivery (EDD). An EDD calculator is commonly used.

According to studies, less than 4% of births occur on the exact due date, 60% occur within a week of the due date, and almost 90% occur within two weeks of the due date.

Hence, a mother can give birth within two weeks of the due date. Plus, it is currently not possible to predict the exact day of birth with the EDD calculator.

What Is An EDD Calculator?

The estimated due date calculator or the EDD calculator is the one that helps you estimate your date of delivery based on your last menstrual period (LMP), ultrasound scans, conception date, or IVF transfer date.

The due date might be estimated by adding 280 days (9 months and 7 days) to the first day of the last menstrual period (LMP).

Pregnancy Detection

Pregnancy can be detected with the help of pregnancy tests. The woman herself can experience a number of symptoms, including a missed menstrual period, increased basal body temperature, fatigue, nausea, and increased frequency of urination.

Pregnancy test kits check your urine or blood for a hormone called human chorionic gonadotropin. | Image Source: pexels 

Pregnancy tests include the detection of hormones that serve as biomarkers for pregnancy. It also includes clinical blood or urine tests that can detect pregnancy from six to eight days after fertilisation.

Clinical blood tests are more accurate and can detect exact amounts of the hormone hCG (the hormone which helps to produce progesterone to maintain the pregnancy) which is only present if you are pregnant. However, they take more time to evaluate and are more expensive than home pregnancy urine tests.

Determining The Estimated Due Date

For determining the estimated due date, follow these steps:

  1. You can determine the first day of your last menstrual period.
  1. Count back three calendar months from that date.
  1. Add one year and seven days to that date.

How to Calculate Your Due Date?

We have the answer to the most common question – “How do I calculate my due date?” Here’s how:

First Day of Last Period

There are several rules to calculate the due date of a pregnancy by the last menstrual period. Some of the rules are:

  • Naegele’s rule: This is the standard method of calculating the due date. The formula for this rule is – LMP + 280 days.
  • Parikh’s rule: It is used for irregular cycles, and the expected date of delivery is calculated by adding nine months to the last menstrual period, subtracting 21 days, and then adding the duration of previous cycles.

The formula for this rule is – LMP + 280 days – 21 days + the length of previous cycles.

In addition to these, there are Mittendorf-Williams rule and Wood’s rule.

Conception Date

You can also calculate your due date with the help of the conception date. You might think that it is easier to calculate your due date from the date you conceived by simply adding 266 days. But it is a little more complicated.

Even if you know the exact date of your intercouse, the exact date of conception is never known. This is so because sperm can live in the woman’s body for up to five days and the egg can survive for 24 hours after it’s released from the ovary.

Therefore, conception can occur several days after the unprotected intercourse.

An ultrasound scan uses the size of the foetus to determine the gestational age. | Image Source: pexels

Ultrasound Scan

An ultrasound scan uses the size of the foetus to determine the gestational age (the time elapsed since the first day of the last menstrual period).

When a doctor performs an ultrasound, they prepare a report and include two estimated due dates.

The first date is calculated using the date of the LMP while the second date is based on the ultrasound measurements.

IVF Transfer Date

Firstly, find the date the eggs were transferred into your uterus and the type of transmission that was carried out.

If you had a Day-5 embryo transfer, you can add 261 days to the transfer date. Whereas, if you had a Day-3 transfer, you can add 263 days to the transfer date.

Other Methods for Estimating The Gestational Age

Some other methods for estimating the gestational age are:

Clinical Examination

A reliable method for estimating the gestational age is a pelvic examination supported by good menstrual records in the first trimester.

Doppler Ultrasonography

With the development of the foetus in the womb, its heartbeat can be heard using Doppler ultrasound by 10 to 12 weeks in most of the pregnancies. Therefore, the gestational age should be at least 10 to 12 weeks as the foetal heartbeat is heard.

Human Chorionic Gonadotropin Pregnancy Test

Human chorionic gonadotropin first becomes detectable in the expecting mum’s blood and urine between six to 14 days after fertilisation.

Therefore, the gestational age would be at least three to four weeks at the time of a reliable hCG pregnancy test.


When there is a possibility of a mum carrying twins as a result of in vitro fertilisation, the gestational age can be determined from the date of embryo transfer.

Observed Gestational Age at Delivery

The earlier age at delivery of multiple gestations reflects the increased incidence of preterm labour and obstetric intervention for complications such as preeclampsia, placental abruption, foetal growth restriction, and increased risk for stillbirth that occurs as the number of foetuses increases.

A full term pregnancy is considered to be 39 to 40 weeks irrespective of the number of foetuses being carried.

Frequently Asked Questions

Is EDD calculator accurate?

Estimated due date is never 100% accurate. You can consider the due date just as an estimated date of your baby’s arrival.

Can I plan my due date?

As no estimated due date is never 100% accurate, you cannot plan an exact due date. However, you can calculate based on the date of your last menstrual period and your typical cycle length to discover the days you’re most likely to be fertile.

Can my due date change?

Yes, your due date can change for several reasons. Abnormal fundal height, unusual range of alpha-fetoprotein (a protein made by the baby) are some of the reasons.


While the due date can be estimated, the actual length of pregnancy depends on factors, including age, health of the mum, length of previous pregnancies, and the weight of the mum at birth. However, there are some more factors affecting natural variation in pregnancy terms. Your due date can change because of reasons like abnormal fundal height and unusual range of alpha-fetoprotein. It is advisable to talk to your doctor if you have any concerns.

DISCLAIMER: We have taken steps to check the accuracy of information & practices shared above; however, it is not a replacement for a doctor’s opinion. Please check with either your doctor, or an expert, before trying any suggestion, practice, or medication mentioned here.

Also Read: EDD Calculator in Hindi



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