For a day 5 blastocyst transfer, add 261 days to your transfer date to get your estimated due date. For a day 3 transfer, add 263 days. That's the whole formula, the embryo's age at transfer is what makes FET dating more precise than standard last-period based dating, since your clinic knows the exact age of the embryo rather than estimating from an assumed ovulation day.
Why FET Dating Doesn't Use Your Last Period
Standard pregnancy dating counts from the first day of your last menstrual period (LMP), because for most pregnancies, that's the only reliable starting point available. With IVF and FET, you have something more precise, the exact date your embryo was transferred, and its exact age at that point, day 3 or day 5. That precision is why IVF and FET due dates are calculated differently from a typical LMP based pregnancy, not because the underlying biology of the pregnancy is any different.
The Formula Clinics Use
The core idea is simple: start from 266 days (the standard length from conception to due date) and subtract the embryo's age at transfer. A day 5 embryo is already 5 days past fertilization at transfer, so 266 minus 5 equals 261 days added from the transfer date. A day 3 embryo is 3 days past fertilization, so 266 minus 3 equals 263 days from transfer. That's where both numbers come from, they're not arbitrary, they follow directly from how old the embryo already is on transfer day.
Day 5 Blastocyst FET: +261 Days (Worked Example)
If your transfer date was March 1, your estimated due date would be November 17, calculated as March 1 plus 261 days. Our IVF due date calculator runs this instantly for your actual transfer date.
Day 3 Cleavage Stage FET: +263 Days (Worked Example)
Using the same March 1 transfer date, but for a day 3 transfer instead, your estimated due date would be November 19, two days later than the day 5 example, calculated as March 1 plus 263 days. The 2 day difference directly reflects the embryo being 2 days younger at transfer.
Fresh vs Frozen: Why Storage Time Doesn't Change the Due Date
This is worth being clear about, since it's a genuinely common point of confusion: whether your embryo was transferred fresh or after being frozen for weeks, months, or even years, the due date calculation is identical. What matters is the embryo's developmental age at transfer, day 3 or day 5, not how long it spent frozen beforehand. Storage time has no bearing on the math at all.
Equivalent LMP: Counting Back 19 or 17 Days
If your OB's office uses standard LMP based dating for scheduling and charting, your clinic may give you what's called an equivalent LMP, a date that, if treated as a normal last period, would produce the same due date your IVF dating already calculated. For a day 5 transfer, that's your transfer date minus 19 days. For a day 3 transfer, it's your transfer date minus 17 days. Using the March 1 example above, the equivalent LMP would be February 10 for a day 5 transfer, or February 12 for a day 3 transfer. This is purely a calendar conversion for handoff between your fertility clinic and your regular OB, it doesn't change your actual due date.
How Many Weeks Pregnant Are You on Transfer Day, or at Beta?
In gestational age terms, which count from that equivalent LMP rather than from conception, you're 2 weeks 5 days pregnant on the day of a day 5 transfer, and 2 weeks 3 days on the day of a day 3 transfer, because gestational age counts from the equivalent LMP (19 or 17 days before transfer). This can feel strange at first, since the actual embryo is only days old, but it's the same gestational age convention used throughout standard pregnancy dating, just applied to a timeline where the starting point is known precisely instead of estimated.
By the time of a typical beta hCG blood test, usually around 9 days after a day 5 transfer or 11 days after a day 3 transfer, you're generally about 4 weeks pregnant in gestational age terms either way. The two timings land on almost the same gestational week because clinics typically schedule beta testing around the same point in embryonic development, regardless of which day the embryo was transferred.
Day 6 Transfers and Clinic Variation
Day 6 blastocyst transfers are less standardized than day 3 or day 5 across different clinics. Some clinics date a day 6 transfer the same as day 5 (adding 261 days), while others use a slightly adjusted figure. If you had a day 6 transfer, the most reliable approach is confirming directly with your clinic which convention they used, rather than assuming either the day 5 or day 3 formula applies exactly.
IVF/FET Date vs LMP Apps, Why They Disagree
If you've entered your dates into a general pregnancy app that only supports LMP based input, you may see a due date that doesn't quite match your clinic's IVF based calculation. This is usually just a result of the app not having a dedicated IVF or FET mode, so it's working from an estimated or incorrectly entered LMP rather than your actual, known transfer date. Your clinic's IVF based due date, calculated from your exact transfer date, is generally the more precise of the two.
When Ultrasound May Still Adjust Care Plans
Even with a precisely dated IVF or FET pregnancy, your clinic may still use an early ultrasound to confirm growth is on track and occasionally refine the estimated due date slightly, the same way any pregnancy's dating can be adjusted based on direct measurement. This isn't a sign anything is wrong with the original IVF based calculation, it's simply standard practice to confirm with direct observation once it's available. To compare scan-based dating with your transfer date, use the Ultrasound Due Date Calculator.
Use the Free IVF/FET Due Date Calculator
Rather than doing this math by hand, our IVF due date calculator handles day 3, day 5, fresh, and frozen transfers with your exact transfer date, and also gives you your equivalent LMP and current gestational age alongside your due date.
Frequently Asked Questions
Is FET dating the same as fresh IVF dating?
Yes, mathematically identical. Both use the embryo's age at transfer, day 3 or day 5, regardless of whether the embryo was transferred fresh or after being frozen.
Is a Day 6 blastocyst transfer due date calculator the same as Day 5?
Not necessarily, practice varies by clinic. Some use the same day 5 formula for day 6 transfers, others adjust slightly. Confirm with your clinic which convention they used for your specific transfer.
Why does my due date from IVF dating differ slightly from what a pregnancy app shows?
Most general pregnancy apps don't have a dedicated IVF or FET input mode, so they're often working from an estimated or approximate LMP rather than your actual transfer date. The IVF based calculation, using your exact transfer date, is generally more precise.
What is an equivalent LMP and why do I need one?
It's a calculated date that lets your OB's office use standard LMP based scheduling and charting systems, even though your actual pregnancy was dated by transfer date, not a last period. It's a conversion for convenience, not a change to your real due date.
Is an IUI due date calculated the same way as IVF or FET?
No, IUI (intrauterine insemination) dating typically follows standard LMP or ovulation based counting instead, since IUI doesn't involve a known embryo transfer date or embryo age the way IVF and FET do. If you conceived via IUI, LMP based or ovulation based dating, rather than the transfer-date formulas on this page, is the more accurate approach for you.
Medical disclaimer. This article is for general educational purposes and is not a substitute for medical advice. Your fertility clinic's calculated due date, based on your specific case, should always take priority over a general calculator. Consult your healthcare provider for guidance specific to your situation.