IVF due dates are calculated from a known conception date, making them more precise than due dates based on a last menstrual period. The core formula is simple: take your transfer date, subtract the age of your embryo to find the conception date, then add 266 days. That gives you your estimated due date at 40 weeks.
Why IVF Dating Is More Accurate
In a natural pregnancy, doctors estimate the due date from the first day of your last menstrual period (LMP) and assume you ovulated 14 days later. That assumption is often wrong. Cycles vary, ovulation can happen earlier or later, and many women don’t remember their exact LMP. All of that introduces guesswork.
With IVF, there’s no guesswork. You know the exact date of egg retrieval (which equals ovulation) and the exact date the embryo was transferred. The American College of Obstetricians and Gynecologists (ACOG) states that when a pregnancy results from assisted reproductive technology, the ART-derived gestational age should be used to assign the estimated due date. In other words, your transfer date takes priority over ultrasound dating, which is a reversal of the usual approach for natural pregnancies.
The Formula for Day 5 Transfers
Most IVF transfers today use Day 5 blastocysts. Here’s the math:
- Conception date = transfer date minus 5 days (because the embryo was fertilized 5 days before transfer)
- Estimated due date = conception date plus 266 days
A shortcut: just add 261 days to your Day 5 transfer date. ACOG confirms this directly, stating that for a Day 5 embryo, the estimated due date is 261 days from the embryo replacement date.
So if your Day 5 transfer was on March 1, your conception date is February 24, and your due date falls on November 17 (266 days from February 24, or equivalently, 261 days from March 1).
The Formula for Day 3 Transfers
If you had a Day 3 embryo transfer, the same logic applies with a small adjustment:
- Conception date = transfer date minus 3 days
- Estimated due date = conception date plus 266 days
The shortcut here: add 263 days to your Day 3 transfer date. The only difference between a Day 3 and Day 5 calculation is how far back you count to reach the conception date.
Fresh Transfers vs. Frozen Transfers
The calculation works the same way for both fresh and frozen embryo transfers. What matters is the age of the embryo at the time it was transferred, not whether it spent time in a freezer. A Day 5 frozen embryo transfer (FET) uses the same 261-day formula as a Day 5 fresh transfer. A Day 6 or Day 7 frozen transfer would use 260 or 259 days, respectively.
If you used donor eggs, the same rules apply. The embryo age at transfer is still the starting point. The donor’s retrieval date doesn’t change your formula, because what you need is the number of days between fertilization and transfer, which is already captured by the embryo’s age designation (Day 3, Day 5, etc.).
How to Calculate Your “Corrected” LMP
Here’s where things get practically useful. Most pregnancy apps, OB offices, and gestational age wheels are built around a last menstrual period. Since you didn’t conceive through a normal cycle, you need a “corrected LMP” to plug into those tools.
The formula: take your egg retrieval date (or the date of fertilization, which is the same day) and subtract 14 days. That gives you an artificial LMP that aligns your IVF pregnancy with the standard 40-week calendar. Textbooks assume ovulation happens exactly 14 days after the start of a period, so by backing up 14 days from your known ovulation, you create a date that makes all the standard tracking tools work correctly.
For example, if your eggs were retrieved on January 20, your corrected LMP is January 6. Enter January 6 as your “last period” in any pregnancy app, and the weeks, milestones, and due date will all line up.
If you had a frozen transfer and don’t remember your retrieval date, you can reverse-engineer it. Subtract the embryo age from your transfer date to get the fertilization date, then subtract 14 more days for the corrected LMP.
When Your OB’s Date Doesn’t Match
Some women find that their OB’s office assigns a slightly different due date after a first-trimester ultrasound. In natural pregnancies, an early ultrasound measuring crown-rump length is considered the most reliable dating method when the LMP is uncertain. But ACOG’s guidance is clear that IVF pregnancies should be dated from the transfer, not from ultrasound measurements.
Small discrepancies of a few days between your IVF-calculated date and an ultrasound measurement are normal and don’t indicate a problem. Embryos grow at slightly different rates in the first weeks. If your provider tries to change your due date based on an early ultrasound, it’s worth mentioning that your dates are based on a known transfer date. Most providers will defer to the IVF calculation once they’re aware of it.
Quick Reference by Transfer Type
- Day 3 transfer: add 263 days to your transfer date
- Day 5 transfer: add 261 days to your transfer date
- Day 6 transfer: add 260 days to your transfer date
- Corrected LMP: transfer date minus embryo age minus 14 days
These formulas work identically for fresh cycles, frozen cycles, and donor egg cycles. The only variable you need is the date of your transfer and the age of the embryo on that day.