Pregnancy Calculator
Calculate your due date, current week of pregnancy, and key milestones based on your last period or conception date
How to use:
- • Enter the first day of your last menstrual period for the most common calculation method
- • Pregnancy is calculated from LMP, not conception — that's why you're "2 weeks pregnant" at conception
- • Use the ultrasound method for the most accurate dating if you have scan results
- • Due dates are estimates — only 5% of babies are born on their exact due date
- • Always confirm dates with your healthcare provider
What this calculator estimates — and who uses it
This pregnancy calculator estimates your estimated due date (EDD), current gestational age in weeks and days, and upcoming developmental milestones. It is used by people who are newly pregnant and want a quick reference before their first prenatal appointment, as well as by those tracking week-by-week progress throughout a pregnancy. Partners, family members, and doulas also use it to stay informed. Because the calculator relies on the dates you enter, results are estimates — not medical diagnoses. Always confirm your due date and gestational age with a qualified OB-GYN or certified midwife.
How your due date is calculated: Naegele's rule
The most widely used formula in obstetrics is Naegele's rule, developed in the early 1800s and still the clinical standard today. The rule is straightforward:
The formula assumes a regular 28-day menstrual cycle and that ovulation — and therefore conception — occurs around day 14 of that cycle. This is why gestational age is measured from the LMP rather than from the moment of conception: the LMP is a concrete, observable date, while conception is rarely witnessed directly.
A practical consequence of this convention is that at the moment of conception you are already considered 2 weeks pregnant. By the time most people take a home pregnancy test — around week 4 — the embryo has already been developing for about two weeks. This counting method is used universally by obstetric providers, so any week-by-week pregnancy app or book you use will match these numbers.
Three ways to date a pregnancy
1. Last menstrual period (LMP)
Enter the first day of your last period and the calculator adds 280 days. This is the default method at most first prenatal visits because it requires no additional testing. Accuracy depends on having a regular cycle — if your cycle is consistently longer or shorter than 28 days, the estimate shifts accordingly.
2. Conception date
If you tracked ovulation (via ovulation predictor kits, basal body temperature, or fertility monitoring) and have a reliable conception date, the calculator adds 266 days (38 weeks) to that date. The difference from the LMP method — 280 vs. 266 days — accounts for the roughly 14 days between the start of the last period and ovulation.
3. First-trimester ultrasound (most accurate)
During an early ultrasound (ideally between weeks 8 and 13), the sonographer measures the crown-rump length (CRL) of the embryo or fetus. Because growth at this stage is highly predictable across pregnancies, CRL measurement can establish gestational age with an accuracy of ±5 to 7 days — considerably better than LMP dating. If ultrasound dating differs from LMP dating by more than about a week in the first trimester, many providers will revise the due date to match the ultrasound.
Worked example
LMP = September 10, 2025. Adding 280 days gives a due date of June 17, 2026. At an 8-week ultrasound on November 5, 2025, the CRL matches 8 weeks exactly — confirming the LMP-based date. The estimated conception window was around September 24, 2025 (LMP + 14 days).
How accurate is a due date?
Research consistently shows that only about 4–5% of babies are born on their exact estimated due date. Roughly 80% arrive within two weeks on either side of that date. The EDD is therefore best understood as the midpoint of a normal birth window — not a deadline or a precise prediction.
The American College of Obstetricians and Gynecologists (ACOG) defines gestational age ranges as follows:
Babies born before 37 weeks are considered preterm, which carries increased risks of respiratory, feeding, and developmental challenges. Each additional week of gestation between 34 and 37 weeks meaningfully reduces those risks.
Pregnancy is divided into three trimesters: the first (weeks 1–13) covers critical embryonic organ formation; the second (weeks 14–26) is typically the most comfortable and includes the anatomy scan around week 20; the third (weeks 27–40) involves rapid growth, lung maturation, and preparation for birth.
Things worth knowing about due dates
- •Irregular cycles reduce LMP accuracy. If your cycles are consistently longer than 28 days (for example, 35 days), ovulation likely occurs around day 21, and your due date will be about a week later than the standard formula suggests. Early ultrasound is especially helpful in this situation.
- •Early ultrasound dating is the most reliable anchor. An 8–10 week ultrasound is the gold standard for establishing gestational age. If your LMP and ultrasound disagree by more than 5–7 days, most providers will revise the EDD based on the ultrasound.
- •The due date is a midpoint, not a deadline. Spontaneous labor between 39 and 41 weeks is considered completely normal. Provider policies on monitoring and induction typically begin to change around 41–42 weeks.
- •Due dates can be officially revised. If a later ultrasound — particularly in the second trimester — shows a significant size discrepancy, your provider may adjust the EDD. However, second-trimester dating is less precise than first-trimester dating.
- •Maintain overall health throughout pregnancy. Nutrition, hydration, and prenatal vitamins all matter. You may also find our daily water intake calculator and ideal weight calculator useful for tracking related health metrics during pregnancy.
❓Pregnancy calculator FAQ
🔗Related Tools
Age Calculator
Calculate age between two dates
Ideal Weight Calculator
Calculate ideal body weight using multiple medical formulas
Water Intake Calculator
Calculate daily water intake needs based on weight, activity, and climate
Tip: Explore our complete toolkit to find more tools that can help with your workflow. Each tool is designed to work seamlessly with others for maximum productivity.
Disclaimer: A2ZKit's tools, calculators, cheat sheets, and articles are provided for general information and educational purposes only, on an "as is" basis without warranties of any kind. They are not financial, investment, tax, accounting, medical, health, or legal advice, and are not a substitute for a qualified professional. Results may be inaccurate or incomplete — verify independently and consult an appropriate professional before making any decision. Some tools process files — such as PDFs and images — entirely in your browser; you are responsible for keeping your own backups, and we are not liable for any data loss, file corruption, or inaccurate output. You use A2ZKit entirely at your own risk. By using the site you agree to our Terms of Service.