Pregnancy Calculator — Due Date & Week-by-Week Timeline
Find your estimated due date based on last menstrual period. See a week-by-week pregnancy calendar with milestones.
Pregnancy Calculator
Quick Answer
If your last menstrual period (LMP) began on a given date, your estimated due date is 40 weeks (280 days) later — calculated by adding 9 months and 7 days, or simply adding 280 days. First trimester: weeks 1–12. Second trimester: weeks 13–27. Third trimester: weeks 28–40. Only about 5% of babies are born on their exact due date.
How the Pregnancy Calculator Works Step by Step
A pregnancy calculator estimates your due date (EDD — Estimated Due Date) and current gestational age based on the first day of your last menstrual period (LMP). Naegele's Rule — the standard clinical method — adds 280 days (40 weeks) to the LMP date, or equivalently, adds 9 months and 7 days. This assumes a 28-day menstrual cycle with ovulation occurring on day 14; for cycles longer or shorter than 28 days, adjust the due date by the difference.
Gestational age is measured from the first day of the LMP, not from conception (which typically occurs ~14 days after LMP). This means pregnancy is technically 2 weeks "old" at the time of conception — medical professionals and ultrasound reports use this convention universally. A positive pregnancy test typically appears at 4–5 weeks gestational age (2–3 weeks post-conception).
Due date accuracy: ultrasound dating in the first trimester (7–13 weeks) is the most accurate method for confirming gestational age, accurate to ±3–5 days. LMP-based calculation is accurate to ±7–14 days for women with regular cycles. Second-trimester ultrasound is accurate to ±10–14 days; third-trimester is less accurate. About 80% of births occur within 2 weeks before or after the calculated due date.
Understanding Each Pregnancy Calculator Input Field
Each field in the Pregnancy Calculator serves a specific purpose. Here's why each input matters and how to provide the most accurate values:
Last Menstrual Period (LMP)
The first day of your most recent menstrual period before becoming pregnant. This is the standard clinical reference date for gestational age calculation.
Cycle Length
The typical length of your menstrual cycle in days. Standard is 28 days; adjust if yours is consistently longer or shorter, as this affects when ovulation (and conception) likely occurred.
Conception Date (alternative)
If you know your conception date from IVF, ovulation tracking, or other methods, you can calculate EDD as conception date + 266 days (38 weeks from conception = 40 weeks from LMP).
Pregnancy Calculator Formula and Methodology Explained
The Pregnancy Calculatoruses the following validated formula. Understanding the math helps you interpret results accurately and trust the calculations you're relying on.
How the Pregnancy Calculator Formula Works
The 280-day standard is based on the average human gestation period of 40 weeks from LMP. Naegele's Rule adds exactly 9 months and 7 days by calendar arithmetic. The cycle length adjustment recognizes that women with longer cycles (e.g., 35 days) ovulate later (day 21 instead of day 14), pushing conception and thus the due date later by the same number of days.
When to Use the Pregnancy Calculator
- →Immediately after a positive pregnancy test to determine gestational age and estimated due date
- →Planning prenatal appointments and screening tests that are gestational age-dependent
- →Understanding which trimester you're in and tracking pregnancy milestones
- →For IVF pregnancies, calculate EDD from egg retrieval or transfer date using adjusted formulas
💡 Expert Tips for Using the Pregnancy Calculator Accurately
An early ultrasound (7–10 weeks) provides the most accurate gestational age and due date — providers often adjust the LMP-based due date based on fetal measurements.
Prenatal care should begin at 8–10 weeks ideally — the first trimester screening for chromosomal abnormalities (nuchal translucency + blood tests) is typically done at 11–14 weeks.
Important first trimester milestones: heartbeat detectable by transvaginal ultrasound at 6–7 weeks; morning sickness typically peaks at 8–10 weeks and resolves by 14 weeks for most.
Due dates are estimates — full term is defined as 39–40 completed weeks; early term 37–38 weeks; late term 41 weeks; post-term 42+ weeks. Induction is typically offered at 41–42 weeks.
⚠️ Common Pregnancy Calculator Mistakes to Avoid
- ✗Using the conception date as the starting point instead of LMP — this understates gestational age by approximately 2 weeks
- ✗Not adjusting for irregular menstrual cycles — a woman with a 35-day cycle should add 7 extra days to her LMP-based due date
- ✗Treating the EDD as certain — only 5% of babies are born on their exact due date; 80% are born within 2 weeks either side
- ✗Missing gestational age-dependent screenings (NIPT blood test at 10–13 weeks, anatomy scan at 18–20 weeks) by not knowing gestational age accurately
Frequently Asked Questions
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