Calculating your due date – the NFP method is more accurate than the Naegele method
“When is the due date?” – one of the first questions asked after a positive pregnancy test. The answer depends on how the due date is calculated. And there is an important difference here.
How is the due date usually calculated?
Doctors use the so-called Naegele’s rule – a formula that is around 200 years old:
First day of your last period + 7 days – 3 months + 1 year
This calculation is based on a standard 28-day cycle with ovulation on the 14th day. However, as very few people with a uterus have a regular 28-day cycle, this method is often inaccurate.
Calculating the due date based on the presumed date of intercourse is also unreliable, as sperm can survive for up to 5 days and do not indicate the exact day of ovulation.
How is the due date calculated using NFP?
With NFP, ovulation can be precisely determined by monitoring basal body temperature. The calculation is simple:
The day of the first higher temperature reading + 266 days (38 weeks)
The 267th day is the calculated due date.
The rise in temperature indicates the biological start of pregnancy more accurately than the time of sexual intercourse.
Why is a correct calculation important?
An incorrectly calculated due date can lead to unnecessary medical interventions. In the case of very long cycles with late ovulation, the due date is set too early using the Naegele’s rule. This can result in induction being recommended, even though the pregnancy is not yet full-term.
If you have tracked your ovulation using NFP, you can tell your gynaecologist or midwife the exact day of ovulation – this provides certainty when calculating the due date.
Realistic expectations
Even with the most accurate method: Only around 4% of babies are born on the calculated due date. Deviations of two to three weeks are completely normal and natural.
The calculated due date is therefore a guide – not a fixed due date.
Have you tracked your ovulation in the ovolution app? If so, you’ll have all the data you need.