NumberTally How Naegele’s Rule Works
The standard due-date estimate counts 280 days (40 weeks) from the first day of your last menstrual period — even though conception typically happens about two weeks later, around ovulation. That is why pregnancy is dated from the LMP rather than the conception date: the period date is usually known with certainty. When you enter a conception date instead, the calculator adds 266 days (38 weeks); for a day-5 blastocyst transfer it adds 261 days, because fertilization happened about five days before the transfer.
LMP + 280 days = due date
Conception + 266 days · Day-5 IVF transfer + 261 days
Trimesters at a Glance
Pregnancy is divided into three trimesters by gestational week: the first trimester runs from week 1 through week 13, the second trimester from week 14 through week 27, and the third trimester from week 28 until delivery. Most people feel the biggest changes in the first and third trimesters, while the second is often called the “honeymoon” stretch. These are planning guides, not medical advice — your healthcare provider tracks what matters for you.
Frequently Asked Questions
How accurate is a due date calculated this way?
It is an estimate. Only about 1 in 20 babies arrives exactly on the due date; most are born within two weeks before or after. A first-trimester ultrasound can date a pregnancy more precisely, and your provider may adjust the date after one.
Why is pregnancy counted as 40 weeks when conception happens later?
By convention gestational age starts at the last menstrual period, roughly two weeks before ovulation and conception. So at the moment of conception you are already considered about 2 weeks pregnant.
What if my cycles are irregular or I don’t know my LMP?
Date-based estimates are less reliable with irregular cycles. An early ultrasound is the most accurate dating method in that case — ask your provider about one.
Why 261 days for a day-5 IVF transfer?
A day-5 blastocyst was fertilized about five days before the transfer. Adding 266 days from fertilization is the same as adding 261 days from the transfer date.
When should I see a healthcare provider?
As soon as you have a positive pregnancy test. This calculator is a planning aid, not medical advice — your provider confirms the pregnancy and sets your care schedule.
Is my data stored anywhere?
No. Every calculation happens in your browser; nothing is sent to a server or stored.