How an IVF due date calculator works
Most pregnancies are dated with guesswork: doctors count 280 days from the last period and assume day 14 ovulation. After IVF, you get something rare: certainty. Your team knows your embryo's exact age and transfer day, so the American College of Obstetricians and Gynecologists (ACOG) recommends dating from those two facts instead.
The formula is simple. From conception to due date is 266 days; your embryo had already grown for 3, 5, or 6 days before transfer, so you subtract that head start:
- Due date = transfer date + (266 minus the embryo's age at transfer)
- Day 5 blastocyst: transfer date + 261 days
- Day 3 embryo: transfer date + 263 days
- Day 6 blastocyst: transfer date + 260 days
- Fresh cycle dated from retrieval: retrieval date + 266 days
5 day transfer due date vs day 3: why the embryo's age sets the date
Why do a 5 day transfer due date and a day 3 transfer due date use different numbers? Because the clock starts at fertilization, not at transfer. A day 5 blastocyst has already lived 5 of its 266 days, so fewer remain to count. Here is the whole picture:
| Transfer type | How the dating works | Formula |
|---|---|---|
| Day 3 embryo (fresh or FET) | Embryo is 3 days old at transfer, so 263 of the 266 days remain | Transfer date + 263 days |
| Day 5 blastocyst (fresh or FET) | Embryo is 5 days old at transfer, so 261 days remain | Transfer date + 261 days |
| Day 6 blastocyst (fresh or FET) | Embryo is 6 days old at transfer, so 260 days remain | Transfer date + 260 days |
| Fresh cycle, dated from retrieval | Retrieval day stands in for ovulation and conception | Retrieval date + 266 days |
| Natural conception, for comparison | Counted from the last period, assuming day 14 ovulation | First day of last period + 280 days |
The estimates differ by only a couple of days across transfer types. That's normal: your ivf transfer due date is anchored to a known conception, which makes it one of the most reliable ways to date a pregnancy.
FET due date calculator: does frozen change anything?
No, and this surprises a lot of parents. A frozen embryo transfer due date uses exactly the same math as a fresh transfer of the same embryo age. Development pauses completely in storage and resumes at transfer, so a blastocyst frozen on day 5 is still a day 5 blastocyst on transfer day.
To use this page as a FET due date calculator, choose the pill matching your embryo's age when it was frozen (your clinic paperwork will say day 3, 5, or 6) and enter your transfer date. The years in storage don't count toward your pregnancy, even if they counted heavily in your heart.
How many weeks pregnant am I after IVF?
This one catches almost everyone. Gestational age starts about two weeks before conception, at a "last period equivalent." So on the day of a day 5 transfer you are already 2 weeks and 5 days pregnant (2 weeks 3 days for a day 3 transfer), then add one day per day.
That's why a blood test 9 or 10 days after a blastocyst transfer already puts you at about 4 weeks pregnant. It can feel like time travel. The calculator above shows your weeks today, your trimester, and when each changes. Under ACOG's definitions:
- Second trimester begins at 14 weeks 0 days
- Third trimester begins at 28 weeks 0 days
- Full term runs from 39 weeks 0 days through 40 weeks 6 days
A word about the road that got you here
If you're typing a transfer date into a due date calculator, you have likely been through more than most people will ever know: the appointments, the injections, the two-week waits that felt like two-year waits. However long your road was, this date is yours. Circle it gently.
Two practical notes:
- Clinics sometimes refine the date after an early ultrasound. When the numbers disagree, your clinic's date always wins.
- A due date is the middle of a window, not a promise, so hold it warmly but loosely.
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