How Your Due Date Is Calculated
This calculator uses Naegele’s Rule, the standard method in obstetric practice. The formula: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). That 280-day estimate assumes a 28-day cycle with ovulation on day 14.
If your cycle is shorter or longer than 28 days, your actual due date may differ slightly. The calculator adjusts for this automatically when you enter your cycle length. If you know your conception or ovulation date, you can enter that directly for a more precise estimate.
Only about 4% of babies are born exactly on their estimated due date — the “due date” is really the centre of a normal range. Births between 37 and 42 weeks gestation are considered full-term.
Understanding Your Pregnancy Timeline
First trimester (weeks 1–12) — the period of most significant development. The neural tube (brain and spinal cord) forms in weeks 3–4. Heart activity begins around week 6. By the end of the first trimester, all major organs have begun forming.
Second trimester (weeks 13–26) — often called the most comfortable trimester. Movement typically becomes noticeable between weeks 16–22. An anatomy scan is usually performed around week 20.
Third trimester (weeks 27–40+) — the baby gains most of its birth weight during this period. The lungs mature, and the baby moves into the head-down position in preparation for birth, typically between weeks 32–36.
Week-by-Week Size Comparisons
The calculator shows approximate size comparisons for each week — a common way to visualise development that makes the numbers tangible. These are approximate averages; individual variation is significant and normal.
Privacy
Pregnancy dates are sensitive personal information. This calculator processes entirely in your browser — your dates are not sent to any server, not stored in a database, and not connected to any account.
Frequently Asked Questions
How accurate is an estimated due date?
The EDD is accurate within a range. Most healthcare providers consider any birth between 37–42 weeks full-term. Ultrasound measurements in the first trimester are generally more accurate for dating than LMP-based calculation alone, especially if your cycles are irregular.
What if I don’t know my last period date?
Use the conception date option if you know when conception occurred, or enter the date of a positive pregnancy test as an approximate starting point. Your healthcare provider will use ultrasound to confirm gestational age.
When does each trimester start and end?
First trimester: weeks 1–12. Second trimester: weeks 13–26. Third trimester: weeks 27 through delivery.
What does “gestational age” mean?
Gestational age is counted from the first day of your last menstrual period, not from conception. This means pregnancy is technically counted as starting about two weeks before conception actually occurs — which is why a full-term pregnancy is 40 weeks gestation even though fertilisation happens around week 2.
Related Tools
- Ovulation Calculator — track your fertile window and cycle
- TDEE Calculator — caloric needs change significantly during pregnancy
- Habit Streak Tracker — track prenatal vitamins and daily health habits