Everyday Life · Practical Life · Health & Lifestyle
Pregnancy Week Calculator
Calculate your current pregnancy week, trimester, and estimated due date based on your last menstrual period or conception date.
Calculator
Format: YYYY-MM-DD
Format: YYYY-MM-DD
Formula
Gestational age in weeks is calculated by subtracting the date of the Last Menstrual Period (LMP) from today's date, then dividing by 7. The estimated due date (EDD) is LMP + 280 days (Naegele's Rule: LMP + 9 months + 7 days). If conception date is known, gestational age = (Today - Conception Date) / 7 + 2 weeks, since conception typically occurs ~14 days after LMP.
Source: Naegele's Rule — American College of Obstetricians and Gynecologists (ACOG) Practice Bulletin.
How it works
Pregnancy duration is measured in gestational weeks, counted from the first day of the last menstrual period (LMP) rather than from the actual day of conception. This convention is used universally in obstetric medicine because the LMP is a reliably known date, whereas conception is harder to pinpoint. A full-term pregnancy spans approximately 40 weeks (280 days) from LMP, which equals about 9 calendar months and 1 week.
The standard formula for gestational age is: Gestational Age = (Today's Date − LMP Date) ÷ 7, giving the result in weeks and days. The Estimated Due Date (EDD) is calculated using Naegele's Rule: add 9 months and 7 days to the LMP, which is equivalent to LMP + 280 days. If you know your conception date rather than your LMP, the calculator automatically adds 14 days (the approximate length of the follicular phase) to convert it to an equivalent LMP before calculating gestational age. Pregnancy is divided into three trimesters: the first runs from week 1 to the end of week 12, the second from week 13 to week 26, and the third from week 27 until delivery.
This calculator is used by expectant mothers to track their progress, plan prenatal appointments, and understand developmental milestones. Healthcare providers use gestational age to time key screenings — first-trimester nuchal translucency ultrasounds are recommended at weeks 11–13, the anatomy scan at weeks 18–20, and glucose tolerance testing around week 24–28. Knowing your exact week also helps contextualize symptoms, as nausea typically peaks in weeks 8–10, and fetal movement (quickening) is usually first felt between weeks 16 and 25.
Worked example
Example: Calculating gestational age from LMP
Suppose today is July 15, 2025 and your last menstrual period began on January 14, 2025.
Step 1 — Count the days elapsed:
From January 14 to July 15 = 182 days.
Step 2 — Convert to weeks and days:
182 ÷ 7 = 26 weeks and 0 days.
Step 3 — Identify the trimester:
Week 26 falls in the second trimester (weeks 13–26).
Step 4 — Calculate weeks remaining:
40 − 26 = 14 weeks remaining until the estimated due date.
Step 5 — Calculate Estimated Due Date (Naegele's Rule):
January 14 + 280 days = October 21, 2025.
Step 6 — Calculate percentage complete:
182 ÷ 280 × 100 = 65.0% complete.
So at this point, you are 26 weeks and 0 days pregnant, in the second trimester, with approximately 14 weeks remaining until your due date of October 21, 2025.
Limitations & notes
This calculator provides an estimated gestational age and due date based on a standard 28-day menstrual cycle with ovulation on day 14. Women with irregular cycles, polycystic ovarian syndrome (PCOS), or cycles significantly shorter or longer than 28 days may have a gestational age that differs from the calculator's result. In these cases, an early ultrasound (ideally at 8–10 weeks) is the most accurate method for confirming gestational age. Naegele's Rule was derived from the assumption of a standard cycle and is a statistical average — only about 5% of babies are born on their exact due date, with most full-term deliveries occurring between weeks 37 and 42. This tool is intended for informational and planning purposes only and does not constitute medical advice. Always confirm your due date and gestational age with a qualified healthcare provider. The calculator does not account for multiple pregnancies (twins, triplets), IVF transfer dates, or post-dates pregnancies beyond 42 weeks.
Frequently asked questions
How is pregnancy measured in weeks — from conception or from LMP?
Pregnancy is officially measured from the first day of your Last Menstrual Period (LMP), not from conception. This means at the moment of conception (typically around week 2), you are already considered 2 weeks pregnant by medical standards. This convention is used because the LMP is a known, consistent date, whereas pinpointing conception is difficult.
What if I don't know my LMP date?
If you don't know your LMP, you can use your conception date instead — this calculator converts it automatically by adding 14 days. Alternatively, an early ultrasound performed between 8 and 10 weeks of gestation is considered the gold standard for establishing gestational age when the LMP is unknown or uncertain.
When does each trimester start and end?
The first trimester runs from week 1 through the end of week 12 (approximately 0–13 weeks). The second trimester covers weeks 13 through 26. The third trimester begins at week 27 and continues until delivery, with the full-term period defined as weeks 39–40. Preterm birth is defined as delivery before 37 completed weeks.
How accurate is the Naegele's Rule estimated due date?
Naegele's Rule gives a statistically based estimate. Research shows that only about 4–5% of women deliver on their calculated due date, but roughly 80% deliver within 2 weeks of it (between weeks 38 and 42). The estimate is most accurate for women with regular 28-day cycles. Early ultrasound dating is more precise, especially if the LMP is uncertain.
What is considered full term in pregnancy?
According to ACOG guidelines, a pregnancy is classified as: early term (37–38 weeks), full term (39–40 weeks), late term (41 weeks), and post-term (42 weeks or more). Babies born before 37 weeks are considered preterm. The 39–40 week window is considered optimal, as organs including the brain and lungs complete critical development in the final weeks.
Last updated: 2025-01-15 · Formula verified against primary sources.