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Pregnancy Due Date & Week Calculator
One tool for dating a pregnancy, whichever date you have: your last period, an embryo transfer, a scan report, or a due date a doctor has already given you.
How to use it
- Pick the option that matches the date in front of you. Only the fields that option needs are shown, so nothing irrelevant has to be filled in.
- For a last period, use the first day of proper bleeding — spotting the day before does not count — and set your usual cycle length if it is not 28 days.
- For IVF, use the date of the transfer and the embryo day from your embryology report. A frozen transfer is dated exactly like a fresh one of the same embryo age.
- For a scan, enter the date of the scan and the weeks and days printed on the report, not the due date the report calculated.
- If a doctor has already given you a due date, choose that option. The date is kept as given and only the pregnancy week is worked out.
What the estimate can and cannot tell you
- Dating from a last period assumes you ovulated about 14 days before your next period was due. That holds for most regular cycles but is an average, not a measurement — and it is least reliable with irregular cycles, PCOS, or a recent stop of hormonal contraception.
- A first-trimester ultrasound is generally the most accurate way to date a natural conception, because early growth varies little between pregnancies.
- After IVF, dating from the embryo age is more precise still, because fertilisation and embryo age are known facts from the laboratory rather than inferences. ACOG treats ART-derived dating separately from ultrasound dating for exactly this reason, so an IVF date is not automatically overridden by a scan.
- Whether to change an established due date is a clinical decision made by your doctor, using the whole picture. This calculator never revises a date for you.
- Twins are dated the same way, but are usually delivered earlier than the calculated date. The timing is planned individually by your obstetrician and depends on the type of twins.
After IVF: your first few weeks
Continue every medicine exactly as prescribed until your doctor stops it — luteal support is not optional in an IVF pregnancy, and is commonly continued to around 10 to 12 weeks. Attend your beta-hCG blood tests on the dates given, and your first viability scan at around 6 to 7 weeks, which confirms the pregnancy is inside the uterus, how many sacs there are, and the heartbeat. Call the hospital straight away for heavy bleeding, severe one-sided pain, breathlessness, or rapid abdominal swelling after retrieval.
Clinical details & calculation method
- LMP: EDD = LMP + 280 days, adjusted by (cycle length − 28). Gestational age = days since LMP.
- Ultrasound: EDD = scan date + (280 − gestational age on the report in days). Days on the report must be 0–6.
- Embryo transfer: day-3 transfer + 263 days; day-5 blastocyst + 261 days; day-6 blastocyst + 260 days. Time in storage is not counted.
- Egg retrieval, IUI or a known conception day: + 266 days, treating that day as ovulation.
- Known due date: used as entered. Gestational age = 280 − days remaining to that date.
- Gestational age is shown as completed weeks and days. Trimesters are cut at 14w0d and 28w0d. All arithmetic runs on whole calendar days in your device’s local time zone, computed in your browser at the moment you press Calculate — never baked in when the page was built.
Frequently asked questions
Which method should I choose?
Use the most direct record you have. After IVF, use the transfer date and embryo day. If you have a first-trimester scan report, use that. Otherwise use your last period. If a doctor has already given you a due date, choose that option so nothing is recalculated.
Why is an IVF due date more accurate?
Because the exact moment of fertilisation and the exact age of the embryo are known from the laboratory record. With a natural conception both have to be inferred from period dates, which introduces several days of uncertainty.
Does a frozen transfer change the due date?
No. The time an embryo spends frozen does not count. A day-5 blastocyst transferred after two years of storage is dated exactly like a fresh day-5 blastocyst transferred the same day. That is why the calculator asks for the transfer date and not the freezing date.
My scan says a different number of weeks than this calculator. Which is right?
It depends on how the pregnancy was conceived. For a natural conception, an early scan is usually the more reliable date. In IVF, the transfer-based date is usually kept even if an early scan differs slightly, because the embryo age is a known fact rather than an estimate. Discuss any large discrepancy with your doctor — this calculator will not redate a pregnancy for you.
When is the first scan after IVF?
Usually around 6 to 7 weeks of gestation — roughly three weeks after a day-5 transfer. That scan confirms the pregnancy is inside the uterus, how many sacs there are, and the fetal heartbeat.
What if I do not know my last period date?
Do not guess. A made-up date produces a confident-looking result that is simply wrong. Use a scan report if you have one, or the due date your doctor gave you — both are options above.
Will my baby arrive on the due date?
Probably not. A due date marks 40 completed weeks, and most babies arrive in the weeks either side of it. Only about one in twenty are born on the estimated date itself.
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