Pink Mom MathIVF Due Date vs LMP Due Date: Which Calculation Is Right for You?
If you conceived through IVF, you may see two different due dates — one based on your embryo transfer and one based on LMP. They use different starting points and assumptions. This guide explains both methods so you can understand which applies to your pregnancy and why they may differ.
How LMP-Based Due Dates Work
The LMP (last menstrual period) method is the traditional way of estimating a due date. It uses Naegele's rule: add 280 days (40 weeks) to the first day of your last period. This assumes a 28-day cycle with ovulation on day 14.
The due date calculator uses this method by default. It works well when you have regular cycles and know your LMP date accurately. However, it has a built-in assumption that may not apply to everyone: that conception occurred exactly 14 days after your period started.
How IVF Due Dates Work
When you conceive through IVF, the exact date of embryo transfer is known. This eliminates the guesswork about when ovulation and fertilization occurred. The IVF transfer due date calculator uses a formula based on your transfer date and embryo age:
- Day 3 transfer: Due date = transfer date + 263 days (or equivalently, transfer date minus 3 days to get the "fertilization date," then add 266 days)
- Day 5 transfer: Due date = transfer date + 261 days
- Day 6 transfer: Due date = transfer date + 260 days
The calculation also produces an "LMP equivalent" date — a backdated date that, when plugged into the standard LMP formula, gives the same due date. This is useful because most obstetric software expects an LMP input.
Key Differences at a Glance
| Feature | LMP Due Date | IVF Due Date |
|---|---|---|
| Starting point | First day of last menstrual period | Embryo transfer date |
| Assumes ovulation timing | Yes — assumes day 14 | No — fertilization date is known |
| Accuracy | Good for regular 28-day cycles | Most accurate for IVF pregnancies |
| Affected by irregular cycles | Yes — can be off by days or weeks | No — cycle history is irrelevant |
| ACOG recommendation | Default for natural conception | Preferred for ART pregnancies |
Which Should You Use?
If you conceived through IVF: Use the IVF transfer-based calculation. It is the gold standard for assisted reproduction because the timing is precisely known. ACOG recommends using the embryo transfer date as the primary dating method for IVF pregnancies.
If you conceived naturally: Use the LMP-based due date calculator as your starting point. Your provider will likely adjust the date based on your first-trimester ultrasound if the measurements differ from LMP dating by more than 7 days (per ACOG guidelines).
For a detailed explanation of how due dates are adjusted, read our guide on how due dates are calculated.
Why the Two Methods Sometimes Disagree
If you used fertility medications before IVF, your pre-transfer menstrual period may not follow the typical 28-day pattern. This means the LMP date entered into your chart may produce a due date that is several days off from the transfer-based calculation. Always confirm with your IVF clinic which date should be used, and ask your OB to update the chart accordingly.
A first-trimester ultrasound can serve as a third reference point. For IVF pregnancies, ACOG advises keeping the transfer-based date unless the ultrasound differs by more than 5 days.