Pregnancy & Baby
Due date, conception date, weeks pregnant, baby growth, ovulation window — accurate tools backed by ACOG and WHO clinical guidelines for expecting families.
Pregnancy planning and tracking calculators — due date prediction (Naegele's rule), conception date estimation, weeks pregnant, ovulation day. Each uses the standard obstetric formulas as published by ACOG and used by Indian gynaecologists.
Pregnancy
What's here
Due date — based on last menstrual period (LMP) using Naegele's rule (LMP + 280 days, or LMP + 9 months + 7 days). Conception date back-calculated from LMP. Weeks pregnant from any reference date. Ovulation day for cycle planning.
These are general estimates — actual delivery typically occurs ±2 weeks around the predicted date. For high-precision tracking and any concerns, ultrasound dating (especially first-trimester) is more accurate than LMP-based calculations.
How the 40 weeks are divided
The first trimester runs from week 1 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 to delivery. Weeks are counted from the first day of the last menstrual period, so the first two of those forty weeks fall before conception — that is a convention, not a mistake in the arithmetic.
The due date itself is a single day inside a range, not a deadline. ACOG's terminology divides the end of pregnancy into early term (37 weeks 0 days to 38 weeks 6 days), full term (39 weeks 0 days to 40 weeks 6 days), late term (41 weeks 0 days to 41 weeks 6 days) and post-term (42 weeks 0 days and beyond). Most births land somewhere across that spread, which is why the calculator gives a date rather than a promise.
When the calculated date should be overruled
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycles run to 32 days you are ovulating around four days later, and the LMP-based due date is roughly four days early; a shorter cycle pushes it the other way. The same applies if the last period was unusually light or the date is uncertain — in both cases the calculation is only as good as its input.
A first-trimester ultrasound is accurate to within about five days and is what a gynaecologist will date the pregnancy by when it disagrees with the LMP figure by more than a week. Once that clinical due date is set, use it and treat the calculator as a way to check where you are in the timeline rather than as a second opinion. Nothing here replaces antenatal care, and any bleeding, severe pain or reduced foetal movement is a reason to call your doctor the same day, whatever the dates say.
Frequently asked questions
How accurate is due date from LMP?
Naegele's rule (LMP + 280 days) assumes a 28-day cycle and ovulation on day 14. For women with regular 28-day cycles it's accurate within ±5-7 days. For irregular cycles or longer/shorter cycles, ultrasound dating in the first trimester is more accurate (within ±5 days even at 7-12 weeks).
When does pregnancy start counting?
Obstetric convention starts pregnancy week count from the first day of LMP, not from conception. So you're 'pregnant' for ~2 weeks before conception actually happens. Total length is 40 weeks LMP-to-due-date, but actual gestation from conception is ~38 weeks.
Should I use these calculators instead of seeing a doctor?
No — calculators are reference tools. For pregnancy management, regular antenatal visits with a gynaecologist are essential. Calculators help you plan and understand timelines but cannot replace clinical assessment, especially for high-risk pregnancies.
What's the most fertile window?
The 5 days before ovulation through ovulation day itself — typically days 9-14 of a 28-day cycle. Sperm survives up to 5 days in the reproductive tract; the egg survives 12-24 hours after ovulation. The ovulation calculator estimates the window based on cycle length.
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