Pregnancy Due Date Calculator: Everything you need to know

Pregnancy Due Date Calculator

Due Date Calculator: You have taken a pregnancy test, and it appears to be positive. Now you may be wondering when exactly your baby will arrive. This is where due date calculators come in.

Due date calculation: How many weeks pregnant am I?

The due date, or estimated date of delivery (EDD), is the approximate date when labor is expected to begin.

As this date is just an estimate, you’ll probably start labor sometime in the two weeks before and after your due date.

In fact, only 1 in 20 people delivers on their estimated due date, which means only 5 percent of babies worldwide are born on their exact calculated date.

Gestational age vs fetal age

There are two ways to measure the age of a baby during pregnancy. To track pregnancy and calculate a due date, gestational age is used.

Gestation is the time between the conception date and birth, or how long a person is pregnant in weeks. Gestational age is measured from the last menstrual period (LMP) — the first day of your last period — to the current date in weeks. 

In general, pregnancies last anywhere from 38 to 42 weeks (or around 280 days). If a baby is born before 37 weeks, they are considered a premature baby.

The other method of measurement is fetal age. While gestational age measures how far along a pregnancy is in weeks, fetal age is the actual age of the growing baby. 

How does the Due Date Calculator work?

Due Date Calculator
Due Date Calculator

You can choose between two methods when calculating your due date with the help of this calculator:

  1. Due date by the last menstrual period. When using this method, the due date is calculated by adding 280 days (or 9 months, the common length of a pregnancy) to the first day of your last menstrual period. Note that this assumption is for a regular 28-day cycle (cycles can vary from 20 to 45 days), and the menstrual period and ovulation are considered to be the first two weeks of pregnancy. As this method is affected by the regularity of your menstrual cycle, it’s not 100-percent accurate.
  2. Due date by conception date (or ovulation). With this calculation, 266 days of pregnancy are added to the date you conceived.

More About Due Date Calculations 


Method 1: Due date by last menstrual period

There are several ways, or rules, to calculate the due date of a pregnancy by the last menstrual period in addition to the standard one used in the calculator. The following rules are modifications of the standard formula to get more precise results, simply because the more information you can add to the calculation, the more accurate the due date may be. 

Read on if you’re curious about how these advanced formulas work in detail and how you can use them to calculate a more precise due date.

1. Naegele’s rule

This is the standard method of calculating a due date for a pregnancy.

The standard formula is as follows:
LMP + 280 days

This rule considers a regular menstrual cycle to be 28 days (it may vary from 20 to 45 days), with ovulation occurring around the 14th day of the menstrual cycle. If your cycle lasts longer, the estimated due date will be later. If you have a shorter cycle, your due date will be earlier.

2. Mittendorf-Williams rule

The Mittendorf-Williams rule is considered more advanced than Naegele’s rule, because the more info you provide, the more accurate the results will be.

Recommended: How to manage Fever and Chills During Pregnancy

This rule is based on a study that showed that first pregnancies tend to be slightly longer (an average of 288 days from LMP), and for subsequent pregnancies, the delivery date is an average of 283 days from LMP.

  1. First, determine the first day of your last menstrual period.
  2. Next, count back 3 calendar months from that date.
  3. Lastly, add 15 days to that date if it’s your first pregnancy, or add 10 days if it’s not your first pregnancy.

Short formulas look like this:

  • For first pregnancies:

LMP – 3 months + 15 days

  • For subsequent pregnancies:

LMP – 3 months + 10 days

3. Parikh’s rule

Parikh’s formula is used for irregular cycles, and the expected date of delivery is calculated by adding 9 months to the last menstrual period, subtracting 21 days, and then adding the duration of previous cycles.

In short, use this formula: 
LMP + 280 days – 21 days + the length of previous cycles
*The average cycle length

Even though this formula is considered a modification of Naegele’s rule, by using it, the risk of any potential errors related to calculating your expected due date is greatly reduced.

4. Wood’s rule

Wood’s method takes into consideration the individual length of the menstrual cycle as well as the number of pregnancies a person has experienced. 

Check: 17 Pregnancy Do’s and Don’ts for Women

1. First you calculate your expected due date.

  • For first pregnancies:

LMP + 12 months – (2 months and 14 days) = EDD

  • For subsequent pregnancies:

LMP + 12 months – (2 months and 18 days) = EDD

2. Then you use the expected due date in the equations below.

  • For cycles longer than 28 days:

EDD + (actual length of cycle – 28 days) = EDD

  • For cycles shorter than 28 days:

EDD – (28 days – actual length of cycle) = EDD

Method 2: Due date by day of conception

You might think that it would be easier to calculate your due date from the date you conceived by simply adding 266 days, but it is a little more complicated than that. Even if you know the exact date you had sex, the exact date of conception is almost never known. Why is that?

Because it can be challenging to determine the exact date of ovulation and thus the date of conception. Sperm can live in the female body for up to 5 days, and the egg can live for 24 hours after it’s released from the ovary.

Therefore, conception can occur several days after you’ve had unprotected intercourse.


EDD – Estimated Due Date 
LMP – Last Menstrual Period (the first day of your last menses)

You can use Pregnancy Due Date Calculator via NSH

Leave a Comment