Due Date Calculator
Estimate your baby's due date from the first day of your last period, plus how far along you are and your current trimester.
Enter the first day of your last menstrual period to estimate your due date.
How to use this calculator
Enter the first day of your last menstrual period (LMP) and your average cycle length in days (the default is 28). The calculator immediately shows your estimated due date, how many weeks and days pregnant you are today, and which trimester you are in. You do not need to know your conception date — the LMP is the standard clinical starting point because it is easier to identify.
How due dates are calculated
Pregnancy is measured from the first day of your last menstrual period rather than from the day of conception. This convention exists because the LMP is a concrete, knowable date, whereas the exact moment of fertilisation is rarely known. By convention, a full-term pregnancy lasts 40 weeks (280 days) from the LMP.
The widely used formula is called Naegele's rule, named after the 19th-century German obstetrician Franz Karl Naegele. It works by taking the LMP, adding one calendar year, subtracting three months, and adding seven days — which is mathematically equivalent to adding 280 days. Most software, including this calculator, simply adds 280 days directly to the LMP date.
Because Naegele's rule assumes a 28-day cycle with ovulation on day 14, women with longer or shorter cycles are expected to ovulate at a different point. This calculator adjusts the due date by the number of days your cycle deviates from 28. A 32-day cycle means ovulation likely occurs around day 18, so the estimate shifts four days later. A 24-day cycle shifts it four days earlier.
Understanding gestational age and trimesters
Gestational age is the official clinical measure of how far along a pregnancy is. It is counted in completed weeks from the LMP, not from conception, so at the moment of fertilisation a woman is technically considered about 2 weeks pregnant in clinical terms.
- First trimester: weeks 1–13. The embryo develops all major organs. This is also when an early dating ultrasound can most accurately confirm the due date.
- Second trimester: weeks 14–27. Growth accelerates; the anatomy scan (around week 20) checks development.
- Third trimester: weeks 28 to birth. The baby gains weight and the lungs mature. Labour and delivery typically occur between weeks 37 and 42.
Worked example
Suppose your last period started on September 1 and your cycle averages 28 days.
- Add 280 days to September 1 → estimated due date: June 8 of the following year.
- If today is November 1, you are 61 days past the LMP → 8 weeks and 5 days pregnant, in the first trimester.
Now suppose your cycle is 35 days instead of 28. The calculator adds 7 extra days (35 − 28 = 7), moving the estimated due date to approximately June 15.
How to interpret your result
Your due date is a statistical midpoint, not a precise prediction. Only a small fraction of births occur on the exact estimated date; the majority happen within a window of roughly two weeks on either side. Healthcare providers use the term estimated due date (EDD) for this reason. A first-trimester ultrasound is considered more accurate than LMP-based calculation alone because it measures the embryo directly. If the ultrasound date differs from the LMP-based date by more than a week, providers often adjust the EDD to match the ultrasound.
Common mistakes to avoid
- Using the wrong LMP date. The calculator needs the first day of your last period, not the last day or the day your period ended. Using the wrong day shifts the entire estimate.
- Assuming a 28-day cycle when yours differs. Always enter your actual average cycle length. Cycles of 21–35 days are all within normal range, but each affects the ovulation estimate.
- Treating the EDD as a deadline. Going a few days past your due date is common and does not automatically mean something is wrong. Stay in communication with your provider.
- Ignoring ultrasound results. If your provider revises your due date based on an ultrasound, use that date — it is more accurate than any LMP calculation.
- Counting from the wrong date after IVF. With in-vitro fertilisation, the transfer date is used to calculate gestational age differently. Your fertility specialist will provide the correct EDD.
These results are estimates for informational purposes only and are not medical advice. Please consult a qualified healthcare provider to confirm your due date and receive personalised care throughout your pregnancy.
How we calculate this
Due dates are estimated using Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). When cycle length differs from the assumed 28 days, the estimate is adjusted by the difference — a 30-day cycle adds 2 extra days. Gestational age in weeks and days is derived from the number of days elapsed since the LMP. Trimester boundaries follow the conventional 1–13, 14–27, and 28+ week divisions. All results are estimates only and are not a substitute for clinical evaluation.
Sources
Frequently asked questions
How is my due date calculated?
The standard method (Naegele's rule) adds 280 days — 40 weeks — to the first day of your last menstrual period. This calculator also adjusts for cycle lengths other than 28 days by adding or subtracting the number of days your cycle differs from 28.
How accurate is an estimated due date?
It is an estimate. Research suggests only about 4–5% of babies are born on their exact due date; most arrive within two weeks before or after. An early first-trimester ultrasound (before 13 weeks) is currently the most accurate way to confirm gestational age and due date.
What if my cycle isn't 28 days?
Ovulation timing shifts with cycle length. For every day your cycle is longer than 28 days, the estimated due date moves one day later; for every day shorter, one day earlier. Enter your actual average cycle length for a more personalised estimate.
What are the trimesters?
Pregnancy is divided into three trimesters. The first trimester runs from conception through about week 13, the second covers weeks 14–27, and the third spans week 28 until birth. The calculator shows which trimester you are currently in based on your LMP.
Can I use conception date instead of LMP?
This calculator uses the last menstrual period (LMP), which is the clinical standard. If you know your conception date, add roughly 14 days to estimate your LMP equivalent, then enter that date. A healthcare provider can confirm gestational age with an ultrasound.
What does "full term" mean?
Full term is generally defined as 39–40 weeks of gestation. Babies born between 37 and 38 weeks 6 days are considered early term, 39–40 weeks are full term, 41 weeks is late term, and 42 weeks or beyond is post-term. Your provider may discuss plans if you approach 41–42 weeks.
How many weeks pregnant am I?
Gestational age is counted in completed weeks from your LMP. If you entered your LMP, the calculator displays how many weeks and days along you are as of today. For example, if your LMP was 10 weeks ago, you are in your 10th week of pregnancy.
Should I rely solely on this calculator?
No. This tool provides a general estimate for informational purposes only and is not a substitute for medical advice. Always confirm your due date with a qualified healthcare provider, who can also use ultrasound measurements for greater accuracy.