How to use
- Choose what to calculate from: last period, conception or IVF transfer — whichever date you know.
- Enter the date. For last period, add your average cycle length; for IVF, choose a day-3 or day-5 embryo.
- You get your estimated due date, how many weeks pregnant you are today, your trimester, days to go and key dates.
This calculator provides general information only and is not medical advice.
How it is calculated
| Starting point | Due date |
|---|---|
| First day of last period (LMP) | + 280 days (40 weeks) — Naegele’s rule, shifted by (cycle length − 28) days if your cycle is not 28 days |
| Conception (ovulation) date | + 266 days (38 weeks) |
| Day-5 embryo (blastocyst) transfer | + 261 days |
| Day-3 embryo transfer | + 263 days |
The IVF rules follow ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (2017), from the American College of Obstetricians and Gynecologists.
Naegele’s rule is often taught as “add one year, subtract three months and add seven days” to the LMP. Because months have different lengths, that shortcut can differ from 280 days by one to three days — an LMP of March 1, 2026 gives December 8 by the shortcut and December 6 by counting 280 days. This calculator counts days.
Pregnancy weeks are counted from the first day of your last period, so you are already about two weeks along at conception.
Trimesters and term (ACOG definitions)
| Stage | Gestational age |
|---|---|
| First trimester | up to 13 weeks 6 days |
| Second trimester | 14w 0d – 27w 6d |
| Third trimester | 28w 0d onward |
| Early term | 37w 0d – 38w 6d |
| Full term | 39w 0d – 40w 6d |
| Late term | 41w 0d – 41w 6d |
| Post-term | 42w 0d and beyond |
Examples
- LMP January 1, 2026, 28-day cycle → due October 8, 2026.
- The same due date comes from conception on January 15, a day-5 transfer on January 20 or a day-3 transfer on January 18.
- On March 15, 2026 that pregnancy is 10 weeks 3 days (first trimester), and early term starts September 17.
- With a 33-day cycle, ovulation is assumed to be 5 days later, so the due date moves 5 days later too.
Things to keep in mind
- A due date is an estimate. Relatively few babies arrive on the exact day; birth anywhere in the term range is normal.
- If an early ultrasound measurement (crown-rump length) differs enough from the LMP date, your provider will change the due date to match the scan. The date your clinic gives you takes priority.
- If your cycles are irregular or you are unsure of your LMP, the LMP method may be off.
- Your obstetrician or midwife should guide your prenatal care and testing schedule.
FAQ
Why are weeks counted from my period and not conception?
Ovulation and conception dates are rarely known exactly, but most people know when their last period started, so that is the international convention. “4 weeks pregnant” is roughly two weeks after conception.
Why is IVF calculated from the transfer date?
The embryo’s age and the transfer date are known precisely, which makes them more accurate than a period date. ACOG recommends using them for pregnancies conceived with assisted reproductive technology.
How do I count down to my due date?
Put your due date into the D-day calculator to see the days remaining at any time, or use the ovulation calculator if you are planning a pregnancy.