What is the difference between an FET due date and a natural-conception due date?

The arithmetic is identical — both use Naegele's rule of LMP plus 280 days. The only difference is that, in an FET pregnancy, the 'last menstrual period' is a synthetic date calculated by subtracting 17 to 20 days from your transfer date, depending on whether the embryo was a day-3 cleavage embryo, a day-5 blastocyst, or a day-6 blastocyst.

Can I just add 261 or 263 days to my transfer date instead of using a calculator?

Yes — those shortcuts give the same answer as Naegele's rule for a 28-day cycle. A day-3 frozen embryo: transfer date plus 263 days. A day-5 frozen blastocyst: transfer date plus 261 days. A day-6 frozen blastocyst: transfer date plus 260 days. A calculator is faster because it also returns your current gestational age in weeks and days, updated live as you type.

My clinic gave me a different due date than the calculator — which is right?

Trust your clinic first. Once you reach 7 to 10 weeks, an ultrasound-based estimate is generally more accurate than the LMP-derived number, especially if your pre-treatment cycles were irregular or your embryo was frozen and thawed. If the clinic's date and the calculator's date disagree in a meaningful way, the clinic will usually revise your official due date to match the scan.

Why does my gestational age say six weeks when the embryo is only four weeks old?

Because pregnancy is conventionally dated from the last menstrual period, not from conception or fertilization. Ovulation typically happens about two weeks after a period starts, so an embryo that is biologically four weeks old is, by obstetric convention, counted as six weeks of gestation. This 14-day offset applies to FET pregnancies in the same way as natural ones.

What if I am not sure whether my frozen embryo was a day-3 or day-5 transfer?

Check your embryo summary report or ask your clinic's embryology team — the day of freezing or warming is on every IVF paperwork. In practice, day-5 and day-6 blastocyst transfers are by far the most common in modern FET cycles, so day-3 transfers are the less likely default unless your clinic specifically told you otherwise.

Frozen Embryo Transfer: Pregnancy Calculator Guide

Naegele's rule, the standard obstetric dating method, calculates a due date by adding exactly 280 days — 40 weeks — to the first day of the last menstrual period, and for a frozen embryo transfer (FET) the same arithmetic applies once you convert your transfer date into an equivalent LMP. Because ovulation in a natural 28-day cycle falls about 14 days after the period starts, an embryo transferred on day 5 of a blastocyst culture lands roughly 19 days before the corresponding 'last period' date, while a day-3 cleavage-stage embryo lands roughly 17 days before it. That is why fertility clinics often quote a frozen embryo due date as 'transfer date plus 261 to 263 days' — both formulas describe the same 38-week developmental window from fertilization, just expressed from a different starting point. Plugging the converted LMP into a Naegele's-rule calculator gives you the same answer the clinic would, plus your current gestational age in weeks and days so you can track each trimester as it begins.

how to calculate pregnancy due date after frozen embryo transfer — Frozen Embryo Transfer: Pregnancy Calculator Guide

How Naegele's Rule Translates a Frozen Embryo Transfer Into a Due Date

Naegele's rule is the dating method obstetricians and midwives have used for more than 200 years, and it remains the standard taught in clinics worldwide. The formula is straightforward: take the first day of the last menstrual period, add 280 days (40 weeks), and you land on the estimated due date. The formula assumes a 28-day cycle with ovulation around day 14, which is why gestational age runs about two weeks ahead of the embryo's actual age since fertilization — at six weeks of gestation, the embryo itself is only about four weeks old from conception.

For natural conception, the LMP is a date you can usually pinpoint from memory or a calendar. For an embryo transfer, there is no 'last period' in the usual sense, because the embryo was created in the lab and placed in the uterus on a known calendar date. To use Naegele's rule, you have to work backward from the transfer date and pretend a period started there. This sounds artificial, but it lines up with how the embryo's biology actually unfolds: a day-5 blastocyst transferred on a known day is at the same developmental point it would be at roughly 19 days after a hypothetical period, so the math produces a clinically meaningful due date that lines up with what your clinic will quote.

Converting Your FET Date to an Equivalent LMP

Different embryo stages call for slightly different conversions, because a day-3 embryo and a day-5 blastocyst are not at the same developmental age on transfer day. The table below summarizes the equivalent LMP offset for the most common frozen embryo stages. All three offsets assume a natural 28-day cycle; longer or shorter cycles move the conversion by the same number of days.

Frozen embryo stage	Equivalent LMP	Approx. due-date shortcut
Day-3 cleavage embryo	Transfer date minus 17 days	Transfer date plus 263 days
Day-5 blastocyst	Transfer date minus 19 days	Transfer date plus 261 days
Day-6 blastocyst	Transfer date minus 20 days	Transfer date plus 260 days

The 'equivalent LMP' you calculate is the date you enter into a Naegele's-rule calculator. The calculator then adds 280 days and returns your estimated due date, plus the gestational age you would be on any given day. If you skipped a fresh IVF cycle and went straight to FET, your clinic may have used a programmed or natural cycle protocol to prepare your uterus; the conversion math is the same, because it depends only on the embryo's developmental age, not on the hormone regimen used around transfer. Some clinics quote a 'transfer date plus 266 days' shortcut for a day-5 blastocyst — that figure assumes a slightly different LMP offset and falls within the same week of uncertainty.

How to Calculate Your Due Date With the Pregnancy Due Date Calculator

Once you have your equivalent LMP date, the rest is a quick two-input calculation. The Pregnancy Due Date Calculator uses Naegele's rule under the hood, so it gives the same answer a clinician would write on your chart.

Convert your FET date into an equivalent LMP using the conversion table above. Write down the calendar date — for example, a day-5 blastocyst transferred on 14 March becomes an equivalent LMP of 23 February (14 March minus 19 days).
Open the Pregnancy Due Date Calculator and click the date field next to 'first day of last menstrual period'.
Pick the converted LMP date from the calendar picker. The tool will instantly update both the estimated due date and the current gestational age, recalculating as you type.
Adjust the cycle-length field if your pre-treatment cycles were not 28 days. The calculator accepts any whole number from 20 to 45; a 30-day cycle pushes the due date two days later, and a 21-day cycle moves it a full week earlier.
Read the two outputs: the estimated due date (a single calendar date) and the gestational age in weeks and days, recalculated live against today's date. Bookmark the page or screenshot the result so you can refer back to it at each prenatal appointment.

Why the Estimate Can Shift by a Week or More

An LMP-based due date is a statistical midpoint, not a guarantee. Only about 4% of babies are born on their exact estimated date, and a healthy term delivery can happen anytime between 37 and 42 completed weeks. For an FET pregnancy specifically, the conversion from transfer date to LMP is itself an approximation: embryo development varies by a day or so even between carefully cultured blastocysts, and the day of implantation can shift the timeline by another 24 to 48 hours.

That is why most clinics default to a first-trimester ultrasound for official dating once you are 7 to 10 weeks pregnant. The crown-rump length measured at that scan is more accurate than an LMP estimate, particularly if your pre-treatment cycles were irregular, if you conceived while breastfeeding, or if you recently stopped hormonal contraception. If the ultrasound date differs from your LMP-based estimate in a meaningful way, your provider will usually adopt the ultrasound date as the new official due date. Treat the calculator result as a working estimate for planning purposes and let your clinic confirm the final number.

A second source of variation is your cycle length. Naegele's rule assumes a 28-day cycle; a 30-day cycle pushes the due date two days later, a 32-day cycle pushes it four days later, and a 21-day cycle moves it a full week earlier. The calculator handles this automatically by adjusting the estimate by (your cycle length minus 28) days, but the move is still small compared with the uncertainty introduced by embryo development and implantation timing. All of this arithmetic is done in UTC epoch-day terms inside the tool, so daylight-saving changes, time-zone travel, or device clock drift will not push the answer by a day.

Using Your Due Date as a Planning Tool, Not a Promise

The real value of a due date is the calendar it gives you for the rest of pregnancy, not the single delivery day. Once you have a number, you can map out roughly when each trimester begins, when standard screening windows open, and when fetal movement is likely to become noticeable.

Gestational milestone	Approximate gestational age	How to read it on the calculator
End of first trimester	13 weeks 0 days	Subtract 27 weeks from your due date
Anatomy or anomaly scan window	18 to 22 weeks	Subtract 18 to 22 weeks from your due date
Viability milestone	24 weeks 0 days	Subtract 16 weeks from your due date
Early-term window opens	37 weeks 0 days	Subtract 3 weeks from your due date
Full term	39 weeks 0 days	Subtract 1 week from your due date
Late term	41 weeks 0 days	Add 1 week to your due date
Post term	42 weeks 0 days	Add 2 weeks to your due date

These milestones are widely used in obstetric practice to schedule appointments, screening, and delivery discussions, but the actual week your body reaches each one can vary by a few days in either direction. The calculator is a planning aid — it helps you anticipate screening windows and conversations with your provider — not a diagnosis, and not a replacement for prenatal care. Confirm every date, and report bleeding, irregular cycles, or uncertainty about your last period to your obstetrician, midwife, or fertility clinic.

Related reading: How to Transfer Stored Data KSP.