How to Calculate Gestational Age and Due Date (EDD)

There are multiple ways to calculate your due date and gestational age. This calculation varies depending on whether you conceived naturally or through IVF. Below is a short explanation of how to calculate GA and EDD based on conception method.

Calculating Gestational Age After Natural Conception
If you conceived naturally, gestational age is usually calculated using your last menstrual period (LMP). Counting begins from the first day of your last menstrual period. This method assumes a roughly 28-day menstrual cycle. Therefore, it may be less accurate with longer or irregular cycles. Additionally, this calculation is independent of the date of sexual intercourse, which may cause some confusion.

For example, if today is 1st June and your LMP was 1st April, your gestational age would be 8 weeks and 5 days.

Calculating EDD From Natural Conception (Naegele’s Rule)
Naegele’s rule is commonly used to estimate the due date from the LMP. There are 4 steps to calculating the EDD using this rule:

1. Identify the first day of your last menstrual period
2. Subtract 3 months
3. Add 7 days
4. Add 1 year

Example:
LMP: 1 April 2026
EDD: 6 January 2027

This method effectively adds 280 days to the first day of the LMP.

Calculating Estimated Due Date After IVF
For IVF pregnancies, the due date is calculated more precisely using the date on which the embryo was transferred into the uterus, as well as the age of the embryo at transfer (such as a day-3 embryo or a day-5 embryo). This method is more accurate because the timing of fertilization is known.

For IVF pregnancies, the due date is calculated based on the embryo transfer date:
– For a day-3 embryo transfer, add 263 days from the transfer date
– For a day-5 embryo transfer (blastocyst), add 261 days from the transfer date

This approach reflects the true biological age of the embryo.

There are many apps and websites such as this https://obwheel.quartertone.net/#ob|ivf that can help calculate your gestational age and due date.

Confirming Gestational Age With Ultrasound
Regardless of how gestational age is initially calculated, it should be confirmed with an early pregnancy ultrasound. First-trimester ultrasound is the most accurate method for confirming gestational age, with measurements compared against the LMP-based estimate. If the ultrasound-based gestational age differs from the calculated age beyond accepted limits, the due date may be adjusted. Accurate dating is important for screening tests, monitoring of fetal growth, and appropriate management regarding delivery timing.
If you are unsure about your gestational age or due date, your obstetrician can help clarify this during your first antenatal visit.