PregnancyPregnancyPregnancy Due Date Calculator (When Will My Baby Be Born?)

Pregnancy Due Date Calculator (When Will My Baby Be Born?)

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As you progress through pregnancy there question that becomes ever-more important is when will my baby be born? Calculating your pregnancy due date – or the expected delivery date – is incredibly exciting and also near-essential if you want to make plans before you welcome your baby.

And if you’re still planning a pregnancy, you might like to know how to calculate your expected due date even before conceiving. This is a great way to plan your pregnancy if you would rather not be pregnant at a specific time of the year, or if you have a preferred month or season to give birth to your baby.

Whatever the reason is, there are several ways to calculate your expected delivery date.

What is an Expected Delivery Date (EDD)?

Also called the due date, your expected delivery date refers to the estimated date for labor to commence for the birth of your child. From the time of conception, most pregnancies last about 38 weeks.

Statistics show that less than 5% of births fall exactly on their expected delivery date. Getting an accurate date largely banks on the exact conception date being known.

Since the exact conception date is not always known, its estimation might be what accounts for the slight “inaccuracy” in calculating exact due dates. Note that a baby born one week before its due date (37 weeks or earlier after conception) is considered premature (as defined by the CDC).

Ways to Calculate Your Due Date

After finding out you’re pregnant, you’ll definitely want to know how far along you are. Your doctor or medical practitioner can check by calculating your baby’s gestational age or fetal age.

Even though pregnancies are generally said to last for nine months, it’s more accurate to define the period in weeks. Your due date is often determined as 40 weeks from the start of your last menstruation. However, the actual gestation is typically about 38 weeks long.

The easiest way to know your due date is if you know exactly when you conceived. However, this can be hard to pinpoint even if you know your fertile window. This is because sperm can remain in your body days after intercourse or if you had any form of artificial insemination such as IUI, and only fertilize an egg days after. This means that you may not have conceived when you expected. Your doctor will probably use the first day of your last period to estimate when you will give birth. If you had IVF or similar fertility treatment to implant an egg, then you know the dates for certain.

First Day of Last Period

Your menstrual cycle officially begins on the first day of your menstruation. Your ovulation day is typically in the middle of your menstrual cycle, and it falls within your fertile window. For example, if you have a 26-day cycle, you ovulate 13 days after the first day of your period.

What is My Fertile Window?

Your fertile window is open for about 7 days:

  • five days leading up to your ovulation day,
  • the ovulation day itself,
  • and one day after it.

Calculating Your Due Date

To calculate your due date, simply add 40 weeks to the first day of your last period. This is the standard method that doctors use to know a person’s due date.

This method assumes that the length of your menstrual cycle is 28 days. A person with a 28-day menstrual cycle will ovulate two weeks (14 days) after the first day of their last period. These two weeks are what accounts for why pregnancy is said to last for 40 weeks, instead of 38 weeks of gestation. Your ovulation period usually falls in the middle of your menstrual cycle, and this period usually lasts for about 7 days.

Date of Conception

If you’ve been using ovulation test strips or some other means to track your ovulation, you can calculate your due date based on the day you think you conceived. To calculate your due date, add 266 days to the date of conception, which is equivalent to 38 weeks. This is not a standard method used by medical practitioners since your conception date might be inaccurate. Since the sperm can survive in the body days after intercourse, you may not be able to pinpoint the day on which your egg was fertilized.

Naegele’s Rule

To calculate your due date using this rule, simply subtract three months from the day of your last period, and add seven days.

Parikh’s Rule

This formula takes into account your exact menstrual cycle length. To calculate using Parikh’s formula, add nine months to the first day of your last menstrual period. After getting that date, subtract three weeks (21 days), and add the number of days in your last cycle.

Ultrasound Scan

Doing a first-trimester ultrasound scan can help determine your estimated due date. This method is used by medical practitioners, especially for women who do not recall the date of their last cycle or who have irregular periods.

Usually, the physician would measure the length of the fetus and other parameters to determine the due date. If the EDD from the ultrasound differs significantly from the NLP estimation, the doctor will typically go with the due date estimated by the ultrasound.

Calculating IVF Due Date

If you had an IVF (in-vitro fertilization), your due date will depend on what day the embryo was retrieved and fertilized. For the day-three embryo transfer, your due date is 263 days after the transfer, while a day-five embryo will be due for delivery 261 days after the IVF transfer date.


Preparing for the birth of your child can be super exciting and fulfilling. Whatever the means of conception, whether naturally or through IVF, you can know when your baby is due following the simple formulas in this article.

Note however that a due date calculated might be off by one or a few days. After confirming your pregnancy with a pregnancy test, it’s best to get an ultrasound done soon after, especially if you don’t have a regular cycle.

Written by

Anna Thornhill
Anna Thornhill
Anna is one of our expert writers and, as a mom of two lovely kids (a daughter and son), she has plenty of practical experience to draw on when writing guides and reviews. Anna writes about techniques she's used both during pregnancy and as a new mother, such as combination feeding, and guides to products that have made feeding and care of her kids a little easier.


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