IVF Due Date Calculation: 3-Day vs 5-Day Transfers

Dating an IVF Pregnancy

For parents undergoing In Vitro Fertilization (IVF), the standard "Last Menstrual Period" (LMP) calculation method is often inaccurate. This is because medications used during IVF can alter the menstrual cycle, and the exact date of fertilization is known precisely.

Why LMP Doesn't Work for IVF

In a natural pregnancy, conception is estimated to happen 14 days after the LMP. In IVF, fertilization happens in a lab. The embryo is then allowed to grow for a few days before being transferred to the uterus.

Because the embryo has already started developing before it enters your body, we need to account for that "head start" when calculating the due date.

The Calculation Formulas

Instead of adding 280 days to the LMP, we do the following:

For a 3-Day Embryo Transfer:

This means the embryo developed for 3 days in the lab.

• Formula: Transfer Date + 263 days
• (266 days average gestation - 3 days age of embryo = 263 days)

For a 5-Day Blastocyst Transfer:

This means the embryo developed for 5 days in the lab.

• Formula: Transfer Date + 261 days
• (266 days average gestation - 5 days age of embryo = 261 days)

Accuracy of IVF Due Dates

IVF due dates are actually more accurate than natural conception due dates because the exact moment of fertilization is known. There is no guessing about when ovulation occurred.

However, just like any pregnancy, the baby will come when they are ready. The due date is still just an estimate for the 40-week mark.

Frozen Embryo Transfer (FET)

The calculation is the same for frozen embryos. The date the embryo was frozen doesn't matter; the "clock" pauses while it is frozen. We only care about the age of the embryo (3-day or 5-day) and the date it was transferred into the uterus.