The Pregnancy Due Date Calculator estimates your due date (EDD) from the first day of your last menstrual period or from a known conception date, then reports where you are today: gestational age in weeks and days, your current trimester, the dates each trimester ends, the full-term date, and how many days remain. Cycle lengths other than the standard 28 days are adjusted for automatically.
How It Works
The calculator uses Naegele's rule, the standard obstetric dating method. Pregnancy is dated from the last menstrual period (LMP), not from conception, because the LMP is a date most people know. A standard pregnancy runs 280 days (40 weeks) from the LMP, with conception assumed on day 14 of a 28-day cycle. If your cycle is longer or shorter, the due date shifts by the difference, because ovulation shifts with it:
If you know the conception date instead, the engine works from the 266 days (38 weeks) from conception to birth and derives the LMP as 14 days before conception:
Input Parameters
| Input | Description | |---|---| | Method | Last menstrual period or conception date | | LMP / conception date | The first day of your last period, or the conception date | | Cycle length | 20 to 45 days; default 28; used only with the LMP method |
Worked Example: LMP Method
Last period began January 5, 2026, cycle length 28 days, and today is March 2, 2026:
- Estimated due date: January 5 + 280 days = October 12, 2026.
- Estimated conception: January 5 + 14 days = January 19, 2026.
- Gestational age today: 56 days along = 8 weeks, 0 days — first trimester.
- Trimester milestones: first trimester ends March 30, 2026 (LMP + 84 days), second ends July 13, 2026 (LMP + 189 days), and the full-term date (39 weeks) is October 5, 2026.
- Days remaining: 280 − 56 = 224 days.
If the same LMP came with a 32-day cycle, every date shifts four days later: the due date becomes October 16, 2026, because ovulation is assumed four days later.
Worked Example: Conception Method
Conception on February 14, 2026: the due date is February 14 + 266 days = November 7, 2026, and the engine derives the LMP as 14 days before conception, January 31, 2026, so gestational age and trimester reporting work identically.
How the Trimesters Are Bounded
The calculator marks the first trimester through 12 completed weeks and the second through 27 weeks, which is why its milestone dates are LMP + 84 days and LMP + 189 days respectively. Full term is reported at 39 weeks (LMP + 273 days). Babies born from 37 to 42 weeks are all within the normal window — only a small minority arrive on the exact due date, so treat the EDD as the center of a range rather than an appointment.
Limitations
The estimate assumes a regular cycle with ovulation mid-cycle; irregular cycles, recent hormonal contraception, and breastfeeding can all shift ovulation and make LMP dating unreliable. An early ultrasound measuring crown-rump length is more accurate and will override LMP dating in clinical practice if the two disagree by more than about a week. The calculator supports the LMP and conception methods only — it has no IVF transfer mode, and it is a planning aid, not medical advice; confirm dating with your care provider.
Frequently Asked Questions
Which method is more accurate?
If you know it exactly — for example through ovulation tracking or fertility treatment — the conception date avoids the assumption that you ovulated on day 14. For most pregnancies the LMP is what is known, and with a regular 28-day cycle the two methods agree.
Why did my due date move when I changed the cycle length?
Because ovulation, and therefore conception, happens later in a longer cycle and earlier in a shorter one. The calculator shifts the due date one-for-one with the difference from 28 days: a 32-day cycle adds four days, a 26-day cycle subtracts two.
What if the date I enter is in the future?
The calculator rejects an LMP or conception date later than today, since gestational age cannot be negative. If your date is in the future because of a time-zone or entry slip, correct the date and run it again.