When Is My Baby Due?
The first question after a positive test is almost always the same: when is my baby due? The answer is your estimated due date — the day your pregnancy reaches 40 weeks — and it sets the calendar for everything that follows: scans, appointments, maternity leave and the nursery. The due date calculator above works it out from the first day of your last period or from your conception date, and shows your trimester dates and key milestones alongside.
This page explains how due dates are calculated, works through examples, and shows what to expect at each stage. Remember that a due date is an estimate, not an appointment: only about 5% of babies arrive on their due date, and most arrive within two weeks either side of it.
How Your Due Date Is Calculated
Human pregnancy lasts about 40 weeks from the first day of the last menstrual period (LMP), or 38 weeks from conception. The standard rule is Naegele’s rule: add 280 days (40 weeks) to the first day of your last period. That date is your estimated due date, often called the EDD. It sounds odd to count two weeks before conception, but the convention exists because most women know when their period started and do not know exactly when they conceived — and ovulation usually happens about 14 days after the period starts.
Worked Example: LMP March 15, 2026
If the first day of your last period was March 15, 2026, add 280 days and your estimated due date is December 20, 2026. Conception most likely happened around March 29, 2026 — 14 days after the period started. Your first trimester ends on June 20, 2026 (13 weeks 6 days), your second trimester ends on September 26, 2026 (27 weeks 6 days), and your third trimester starts on September 27, 2026. Enter the same date in the due date calculator above and it shows all of these dates, plus a progress bar of how far along you are.
Worked Example: LMP January 1, 2026
If the first day of your last period was January 1, 2026, your due date is October 8, 2026. Conception was most likely around January 15, 2026, and your first trimester ended on April 8, 2026. If your last period was June 1, 2026, your due date is March 8, 2027. The maths is always the same: add 280 days to the first day of the last period.
Due Date by Cycle Length
Naegele’s rule assumes a 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, the due date shifts. The calculator adjusts for this: it adds the difference between your cycle length and 28 days to the result. A 35-day cycle pushes the due date a week later; a 24-day cycle pulls it four days earlier.
| Cycle length | Adjustment | Due date for LMP Jan 1, 2026 |
|---|---|---|
| 24 days | 4 days earlier | October 4, 2026 |
| 26 days | 2 days earlier | October 6, 2026 |
| 28 days | No change | October 8, 2026 |
| 30 days | 2 days later | October 10, 2026 |
| 32 days | 4 days later | October 12, 2026 |
| 35 days | 7 days later | October 15, 2026 |
Due Date From Conception
If you know your conception date — from IVF, fertility tracking or a known single date — the calculation is simpler: add 266 days (38 weeks) to the conception date. If you conceived on January 15, 2026, your due date is October 8, 2026, which matches the LMP calculation for a January 1 period exactly. The calculator above lets you choose either method. Conception dating is slightly more accurate when the date is certain, because it skips the assumption about when you ovulated.
Your Pregnancy Milestones
Your due date anchors a whole calendar of milestones. The table below shows them for a last period of March 15, 2026, with a due date of December 20, 2026.
| Milestone | When |
|---|---|
| Dating / first-trimester scan | May 31 – June 20, 2026 (11–13 weeks) |
| First trimester ends | June 20, 2026 (13 weeks 6 days) |
| Anatomy scan | July 19 – August 16, 2026 (18–22 weeks) |
| Glucose screening | August 30 – September 27, 2026 (24–28 weeks) |
| Second trimester ends | September 26, 2026 (27 weeks 6 days) |
| Third trimester starts | September 27, 2026 (28 weeks 0 days) |
| Early term window | November 29 – December 12, 2026 (37–38 weeks) |
| Full term window | December 13 – December 26, 2026 (39–40 weeks) |
The dating scan in the first trimester is the most accurate way to set your due date. An ultrasound before 14 weeks measures the baby and dates the pregnancy to within a few days, and if it disagrees with the period-based date by more than a week, your doctor will usually switch to the scan date. Later scans are less accurate for dating, because babies grow at different rates.
How Accurate Is a Due Date?
Due dates are estimates, and honest ones come with wide margins. Only about 5% of babies arrive on their due date. About 80% arrive within two weeks either side of it, and the rest arrive earlier or later. First babies tend to arrive a little late — on average a few days past the due date — while second and later babies tend to arrive a little earlier. Your doctor will only start talking about induction if you pass 41 weeks, and most providers will not induce before 39 weeks without a medical reason.
- Early term: 37 weeks 0 days to 38 weeks 6 days. The baby is ready, but the brain and lungs are still maturing.
- Full term: 39 weeks 0 days to 40 weeks 6 days. The ideal window for birth.
- Late term: 41 weeks 0 days to 41 weeks 6 days. Monitoring increases; induction is usually discussed.
- Postterm: 42 weeks 0 days and beyond. Induction is normally recommended.
What to Do After You Know Your Due Date
- Book your first prenatal appointment, ideally before 10 weeks.
- Start folic acid if you have not already — 400 micrograms a day is the standard recommendation.
- Check your diet and hydration: your daily water intake rises in pregnancy, and your calorie needs rise by about 300 a day in the second trimester.
- Ask about the dating scan, usually between 11 and 14 weeks.
- Plan maternity leave around the due date, but keep two weeks of flexibility either side.
- Do not buy everything at once — the nursery can wait until the second trimester.
Related tools: your daily water intake in pregnancy and your calorie needs as they rise through the trimesters. The American College of Obstetricians and Gynecologists explains how due dates are set and why they change.
Summary
Keep the reference figures below handy while you plan.
| Milestone | When (LMP March 15, 2026) |
|---|---|
| Dating scan | May 31 – June 20, 2026 |
| Anatomy scan | July 19 – August 16, 2026 |
| Glucose screening | August 30 – September 27, 2026 |
| Early term window | November 29 – December 12, 2026 |
| Full term window | December 13 – December 26, 2026 |
What to remember
- Your due date is 280 days after the first day of your last period, or 266 days after conception.
- Only about 5 percent of babies arrive on their due date — most arrive within two weeks either side.
- A longer cycle pushes the due date later: a 35-day cycle adds seven days.
- An ultrasound before 14 weeks is the most accurate way to date a pregnancy.
- Full term is 39 to 40 weeks; early term is 37 to 38 weeks.
Your checklist
- Enter the first day of your last period, or your conception date, in the calculator above.
- Note your due date, your trimester dates and your milestones.
- Book your first prenatal appointment before 10 weeks.
- Ask about the dating scan at 11 to 14 weeks.
- Have the hospital bag ready by 37 weeks.
Mistakes to avoid
- Counting from the last day of the period instead of the first day.
- Forgetting the cycle-length adjustment when cycles are not 28 days.
- Treating the due date as an appointment — it is an estimate.
- Panicking when the scan changes the date — that is normal and means the dating improved.
- Planning everything for the exact due date with no slack either side.
Frequently Asked Questions
When is my baby due if my last period was March 15, 2026?
December 20, 2026. Add 280 days (40 weeks) to the first day of your last period. Conception most likely happened around March 29, 2026, and your first trimester ends June 20, 2026.
How is a due date calculated?
By adding 280 days to the first day of your last menstrual period (Naegele’s rule), or 266 days to the conception date. The 280-day count includes the roughly two weeks before conception, because ovulation usually happens about 14 days after a period starts.
How accurate is my due date?
It is an estimate. Only about 5% of babies arrive on their due date, and about 80% arrive within two weeks either side. An early ultrasound before 14 weeks is the most accurate way to date a pregnancy, and your doctor may adjust the date after it.
Does my due date change if my cycles are irregular?
It can. The standard calculation assumes a 28-day cycle. If your cycles are longer, ovulation happens later and the due date moves later — a 35-day cycle pushes it a week later. The calculator above adjusts for your cycle length, and an early scan gives the most reliable date.
What does it mean if my baby is due in the early term window?
Early term is 37 weeks 0 days to 38 weeks 6 days. Babies born then are fully formed, but the brain and lungs are still maturing, so full term (39 to 40 weeks) is the ideal window. Your doctor will not usually induce before 39 weeks without a medical reason.
Can I calculate my due date from conception?
Yes — add 266 days (38 weeks) to the conception date. If you conceived on January 15, 2026, your due date is October 8, 2026. This is slightly more accurate than the period method when the conception date is certain, as with IVF.
Disclaimer: This calculator provides estimates for general information only and is not medical advice. Due dates are estimates and only about 5% of babies arrive on their due date. An early ultrasound is the most accurate way to date a pregnancy. Talk to your doctor or midwife about your pregnancy, scans and any concerns.