Most pregnancy calculators start from the first day of your last period and work forward with Naegele's rule. That works well when your cycles are regular, but it is only an estimate built on the assumption that you ovulated on day 14. If you actually know the day conception happened — through a confirmed ovulation date, timed intercourse, ovulation tracking, or especially IVF/ICSI — you can get a much more precise estimated due date by counting forward from conception instead.
This calculator does exactly that. Enter the date you know fertilization occurred, and it adds the textbook 266 days (38 weeks) of a full-term pregnancy to project your baby's due date. It also tells you how far along you are today, which pregnancy week you have reached, and how many days remain until the estimated due date.
How to Use
- Enter your Known Conception Date. This is the single day you believe fertilization occurred. After IVF or ICSI, the clinic gives you this date directly. If you conceived through timed intercourse, use the day you confirmed ovulation.
- Leave Days from Conception to Birth at the default 266. That is the standard 38-week figure from conception to delivery. Only change it if your fertility clinic uses a different convention for your specific embryo stage.
- Set Pregnancy Progress As Of to the date you want measured. It defaults to today, so the moment you open the page you see your current week. Change it to a future checkup date to preview how far along you will be.
- Read the result card. The highlighted Estimated Due Date is your conception-plus-266 date. The card also shows the implied last period date (conception minus 14 days), your current pregnancy week, total days pregnant, and days until the due date.
Because this tool counts from a known event rather than from an assumed ovulation, it is the method fertility specialists prefer for assisted-conception pregnancies. For a natural conception with irregular cycles, the LMP-based calculator may still be the more realistic estimate, and your ultrasound dating will usually be treated as authoritative after the first trimester.
Why 266 Days?
A typical pregnancy is described as lasting about 40 weeks from the last menstrual period, or roughly 280 days. But fertilization does not happen on day 1 of that period — it happens around ovulation, about 14 days later. Subtract those 14 days and the true time from conception to birth is about 266 days, or 38 weeks. That is why adding 266 days to a known conception date and adding 280 days to the last period date land on the same due date: the two methods are two sides of the same coin.
This is also why an ultrasound after IVF routinely confirms a "gestational age" that is two weeks ahead of the embryo's actual age. Clinics date the pregnancy from the assumed LMP, not from conception, even when conception is known exactly.
Examples
Example 1 — Confirmed ovulation in mid-January. A woman confirms ovulation on January 15, 2026. Adding 266 days gives an estimated due date of October 8, 2026. Measured against September 1, 2026, she is 229 days pregnant, in her 32nd week, with 37 days to go. The implied last period was January 1, 2026 — and counting 280 days from that date lands on the same October 8 due date, confirming the math is consistent.
Example 2 — A spring conception. If conception occurred on March 20, 2026, the due date moves forward to December 11, 2026. As of October 25, she is 219 days pregnant (week 31) with 47 days remaining. Notice that a due date in mid-December means a late-autumn baby, which changes hospital-packing and maternity-leave planning.
Example 3 — What the due date really means. Only about 5% of babies are actually born on their exact estimated due date; roughly 80% arrive within two weeks on either side of it. The due date is a midpoint estimate, not a deadline. If your doctor expects a larger baby or you have a history of earlier delivery, those two-week windows are what matter for planning.
Conception Dating vs. LMP Dating
If you know your conception date precisely, counting forward 266 days is more accurate than estimating ovulation from your last period. The difference between the two methods is usually small — a few days — but it grows when cycles are long or irregular. For someone with 35-day cycles, the assumed day-14 ovulation can be off by a week or more, and the LMP method pushes the due date earlier than it should be.
In practice, most obstetricians treat the first-trimester ultrasound as the gold standard once you are past 12 weeks. If the ultrasound agrees with your conception-plus-266 date within a few days, you can be confident in that due date. If it disagrees by more than about a week early on, your provider will update the date based on the scan.
FAQ
How do I calculate my due date from my conception date?
Add 266 days to the day you conceived. That is the textbook 38-week length of a pregnancy from fertilization to birth. Enter your known conception date above and the calculator does the addition for you, also showing your current pregnancy week and days remaining.
Is 266 days the same as 40 weeks?
Yes, effectively. Pregnancy is commonly described as 40 weeks (280 days) from the last menstrual period, but conception happens about 14 days after that period starts. So 280 minus 14 equals 266 days — 38 weeks — from conception. The two methods describe the same delivery date from different starting points.
How accurate is a due date based on conception?
When the conception date is truly known (for example after IVF/ICSI), it is the most accurate starting point available, usually within a day or two. For natural conceptions, the exact conception day is only an estimate because sperm can survive several days before the egg is fertilized. First-trimester ultrasound remains the clinic's reference after 12 weeks.
What if I conceived through IVF?
Use the conception date your fertility clinic provides for your embryo transfer. Day-3 and day-5 transfers are converted to a conception date by the clinic's own convention, and due dates from IVF are rarely changed unless an early ultrasound disagrees by more than a week.
Why does my pregnancy week count two weeks ahead?
Because medicine dates pregnancy from the last menstrual period, not from conception. On your conception date you are already considered two weeks pregnant in gestational terms. That is why a "38 weeks from conception" due date equals "40 weeks from LMP," and why the card shows an implied LMP of conception minus 14 days.
How many days after conception is full term?
A full-term pregnancy is about 266 days (38 weeks) from conception, which clinicians split into early term (37–38+6 weeks), full term (39–40+6 weeks), and late term (41 weeks). Only a small share of babies are born on the exact due date.
Disclaimer
This due date by conception calculator is provided for informational purposes only and is not medical advice. Estimates are based on the standard 266-day (38-week) pregnancy length from conception and your inputs; individual pregnancies vary, and ovulation or conception dates may be imprecise. Your exact due date should be confirmed by a qualified obstetrician, ideally with a first-trimester ultrasound. Do not use this tool to make medical decisions about delivery timing or induction.