In order to calculate the estimated due Date (EDD) based on ovulation, it is required to add 38 weeks (or 266 days) to ovulation date. In order to determine an exact Gestational Age based on ovulation, it is required to calculate the number of days from the last ovulation until the present day +14 days.
Estimated due Date calculation
In order to determine the EDD based on the ovulation date you need to exactly know the egg release date. It can be done in several convenient ways (ovulationdiary.com). The most reliable way is to combine several methods.
If you know the ovulation date it is required to add 266 days, notwithstanding the last menstruation. The ovulation is more exact marker for EDD.
This is how the practical side looks:
Ovulation date – 15.09.2019, it means 15.09.19+266 = 08.06.20
Note (!): The Date determination based on ovulation is one of several methods for the EDD due determination. If several methods (for example, based on the first day of last menstruation, ultrasound testing, by first movements, etc.) shows different days, then a delivery will approximately begin in the time period from the earliest date until the latest date.
For example: if the EDD calculation based on ovulation shows the date 08.06.20, based on the first day of last menstruation showing the date of 10.06, the first ultrasound testing shows 07.06, and by first movements (it is supposed to be exactly 20 weeks) – 11.06. Thus, delivery will start within the period from June 7, 2020 until June 11, 2020.
In addition to the ovulation based estimated due Date calculation, the first movement method is also used. As a rule, a first-born starts to move in the age of 20 weeks.
How to calculate a Gestational Age based on ovulation date
In order to calculate Gestational Age based on ovulation date, it is required to count the number of days in the period starting from today until the ovulation date in your standard first stage of menstruation cycle. The first (follicular) stage starts along with menstruation and ends within the ovulation.
For example, ovulation was on September 15, 2018, the 1st stage duration is 14 days. The calculation is carried out on October 15. Then:
From 15.09 until 15.10 + 14 days = 44 days or 6 weeks and 2 days
Note (!): If a regular cycle lasts 28 days, then the 1st stage duration is 14 days. If a regular cycle lasts 30 days, then the 1st stage duration is 16 days, etc. How can you use the duration of own follicular stage? It is better to use a standard duration, than an individual one. According to the latter the gestational age based on the first day of last menstruation is carried out.
The point is that the period between the cycle start up to ovulation can be different (10-30 days or more) and the gestational age will be as follows:
Example No. 1:
The I stage duration – 16 days (30-day cycle), ovulation on September 15, calculation date October 15, 2018 (15.10.18)
15.10 – (15.09.18–16) = 6 weeks and 4 days
Example No. 2:
The I stage duration – 25 days (39-day cycle), ovulation on September 15, calculation date October 15, 2018 (15.10.18)
15.10 – (15.09.18–16) = 7 weeks and 6 days
In both cases there are 4 embryonic weeks and 2 days, and an obstetric period (if we calculate based on individual indices, but not standard indices) 6 weeks and 4 days; and 7 weeks and 6 days. While based on the standard the period amounts to 6 weeks and 2 days.
For more information about EDD and gestational age calculation peculiarities in irregular cycle, please refer to a separate article (ovulationdiary.com).