Calculate your due date, current week of pregnancy, trimester, and see a complete milestone timeline.
Pregnancy Summary
About the Pregnancy Calculator — Pregnancy Calculator Online
This free pregnancy calculator online estimates your due date, current gestational age, trimester, and key milestone dates using your last menstrual period (LMP) and Naegele's Rule — the standard obstetric formula used by healthcare providers worldwide. Everything runs locally in your browser with no data sent to any server. Expectant parents, midwives, and healthcare students use it for quick, private due date estimates without opening a medical app or sharing personal data.
Knowing your due date helps you plan prenatal appointments, understand which trimester you are in, prepare for key screening tests, and track your baby's developmental milestones. This calculator gives you all of that from a single date entry, including a full milestone timeline showing significant weeks such as the 12-week scan, the 20-week anatomy scan, the viability milestone at 24 weeks, and the full-term threshold at 37 weeks.
How to Use the Pregnancy Calculator
Enter the first day of your last menstrual period (LMP) in the date field. This is the date used by doctors to calculate gestational age.
Optionally adjust the Cycle Length field from the default 28 days to match your average cycle. This shifts the due date earlier or later accordingly.
Click Calculate.
View your estimated due date, current gestational age in weeks and days, which trimester you are in, and how many days remain until the due date.
Scroll through the milestone timeline to see key developmental and medical checkpoints plotted against your dates.
How the Calculation Works
The calculator uses Naegele's Rule, the standard obstetric formula for due date estimation.
Standard Naegele's Rule: Due Date = LMP + 280 days (40 weeks). This assumes a 28-day cycle with ovulation on day 14.
Cycle length adjustment: For cycles other than 28 days, the due date is shifted by the difference: Due Date = LMP + 280 + (cycle length − 28) days. A 32-day cycle shifts the due date 4 days later; a 24-day cycle shifts it 4 days earlier.
Gestational age: Calculated as the number of days from LMP to today, expressed in weeks and days. Doctors count from LMP, not from conception, which is why gestational age is approximately 2 weeks ahead of fetal age.
Tips for Getting the Most Accurate Results
LMP-based calculation is a widely used estimate, but a few factors affect its precision.
Use your most recent cycle's start date: If your cycles are irregular, the LMP date is less reliable as a reference point. Use the first day of the last period that occurred before your positive pregnancy test for the most relevant starting date.
Enter your actual average cycle length: The default of 28 days is an average, but many people have cycles of 25–35 days. Entering your true average (tracked over several months) will give a more personalised due date estimate than using the default.
Confirm with your healthcare provider: LMP-based due dates are starting estimates. Your doctor or midwife will confirm the due date using a first-trimester ultrasound, which measures the embryo directly and is more accurate than calendar calculations, especially if your cycle length is irregular.
Note that only ~5% of babies arrive on the due date: The calculated date is the midpoint of a range. Most births occur between 38 and 42 weeks. Use the due date as a planning horizon, not a fixed target, and discuss your specific situation with your healthcare provider.
Track milestone dates carefully: The milestone timeline shows the dates of important prenatal tests such as the nuchal translucency scan (week 12) and anatomy scan (week 18–20). These have specific booking windows, so use the milestone dates to plan your appointments in advance.
Why Use a Pregnancy Calculator Online
A browser-based pregnancy calculator provides an immediate, private estimate without creating an account, downloading an app, or sharing health data with a third-party service. All calculations happen in your browser using standard date arithmetic — no personal information is stored or transmitted. It is accessible on any device with a browser, from a smartphone in a doctor's waiting room to a desktop at home.
Expectant parents in the early weeks of pregnancy, healthcare students studying obstetrics, and midwives doing quick patient references all benefit from an instant, no-friction tool that provides the same core calculation used in clinical practice.
Frequently Asked Questions about Pregnancy Calculator
Only about 5% of babies are born exactly on their calculated due date. The date represents the midpoint of a range — approximately 80% of births occur between weeks 38 and 42. LMP-based calculation is reliable for regular cycles but less so for irregular ones. A first-trimester ultrasound measuring crown-rump length is generally more accurate for confirming the due date.
Gestational age is counted from the LMP and is the standard used by doctors and midwives worldwide. Fetal age (also called embryonic age) is counted from conception, which typically occurs around day 14 of the cycle — approximately 2 weeks after LMP. At 40 weeks gestational age, fetal age is approximately 38 weeks. This calculator reports gestational age, matching clinical convention.
Enter your average cycle length in the Cycle Length field. The calculator applies the standard adjustment: each day your cycle differs from 28 shifts the due date by one day in the same direction. A 32-day cycle gives a due date 4 days later than the standard calculation; a 25-day cycle gives one 3 days earlier. Tracking your cycle over several months gives the most reliable average to enter.
No. This tool provides an educational estimate based on a standard formula and is not a substitute for professional medical advice. Always confirm your due date and gestational age with your healthcare provider, who will use clinical examinations, ultrasound measurements, and your full medical history to provide personalised guidance for your pregnancy.
Yes, completely free with no account required. The calculator runs entirely in your browser — no personal data is transmitted to any server at any point. You can use it as many times as needed, share the URL with others, or bookmark it for reference throughout your pregnancy without any cost or registration.
The milestone timeline includes key events such as: week 4 (implantation, missed period), week 6 (heartbeat detectable by ultrasound), week 12 (end of first trimester, nuchal translucency scan), week 18 (anatomy scan), week 20 (halfway point), week 24 (viability milestone), week 28 (third trimester begins), week 37 (full term), and week 40 (estimated due date).
Yes. You can enter any historical LMP date to calculate what the due date would have been and what the gestational age was at any point. The calculator will show the due date and milestone timeline for that pregnancy period, which can be useful for medical records, curiosity, or retrospective tracking.
Yes. The calculator works on all modern mobile browsers. The date input field uses your device's native date picker for easy selection, and the results and milestone timeline display clearly on small screens. No app download is required — open it in your mobile browser and use it immediately.