CodeKitHubWhat Is This Tool?
A due date calculator estimates when your baby is likely to arrive, counting 280 days (40 weeks) from the first day of your last menstrual period — a method known as Naegele's rule, used by doctors and midwives worldwide. Because ovulation happens later in longer cycles and earlier in shorter ones, the calculator also shifts the estimate by the difference between your cycle length and the standard 28 days.
The same math answers the question most people actually type into search: "how many weeks pregnant am I?" Pregnancy is counted from the LMP, not from conception, so at the moment of conception you are already considered about 2 weeks pregnant. Only around 4–5% of babies are born exactly on their due date; most arrive within two weeks either side. The due date is a planning anchor, not a promise — and ultimately your doctor will confirm or adjust it based on an early ultrasound.
Why Use It?
- Due date via Naegele's rule (LMP + 280 days), adjusted for your personal cycle length from 21 to 35 days.
- Shows exactly how many weeks and days pregnant you are today, plus your current trimester.
- Calculates the calendar dates of key milestones: NT scan window (weeks 11–13), anomaly scan (weeks 18–22), third trimester (week 28), full term (week 37).
- Free, instant and private — the dates you enter never leave your browser; nothing is uploaded or stored.
- Honest about limits: it's a statistical estimate, clearly labeled as such, with a reminder that only an ultrasound dates a pregnancy reliably.
How to Use
- Pick the first day of your last menstrual period (LMP) in the date field.
- Set your average cycle length in days (default 28; adjustable from 21 to 35).
- Press the calculate button.
- Read your estimated due date, current week + day, trimester, days remaining and milestone dates.
Example
Input
LMP May 1, 2026, cycle length 28 daysOutput
Estimated due date: February 5, 2027May 1, 2026 + 280 days = February 5, 2027. With a 35-day cycle the due date would shift 7 days later, to February 12, 2027, because ovulation happens about a week later in a 35-day cycle.
Pregnancy milestones week by week
These are general reference points used in many countries; your own prenatal schedule may differ — always follow your doctor's or midwife's plan. The table exists so you can put your calculated dates next to real appointments, not replace them.
| Week | Milestone | Why it matters |
|---|---|---|
| Week 4–5 | Missed period, positive home test | hCG is usually detectable in urine from around the missed period. |
| Week 6–8 | First prenatal visit, dating ultrasound | The most accurate window for dating the pregnancy by ultrasound. |
| Week 11–13 | NT scan (nuchal translucency) | First-trimester screening for chromosomal conditions. |
| Week 18–22 | Anomaly scan (detailed ultrasound) | Checks the baby's anatomy organ by organ. |
| Week 24 | Viability threshold | Survival outside the womb becomes possible with intensive care. |
| Week 28 | Third trimester begins | Visit frequency typically increases; glucose screening is done around weeks 24–28. |
| Week 37 | Full term begins | Birth from this point is no longer considered preterm. |
| Week 40 | Due date | Only ~4–5% of babies arrive exactly on this day. |
When this calculator is least reliable
- Irregular cycles — if your cycle length varies by more than a few days, the LMP method can be off by a week or more; an early ultrasound is the reliable way to date the pregnancy.
- Uncertain LMP — if you don't remember the exact first day of your last period, the whole calculation inherits that uncertainty.
- Conception while breastfeeding or right after stopping contraception — ovulation timing is unpredictable in these months.
- IVF pregnancies — dating follows the embryo transfer date, not the LMP; use your clinic's dates.
- This tool is for general reference only. It is not a substitute for professional medical advice, diagnosis or prenatal care — confirm the pregnancy and its dating with your doctor's ultrasound.
Where this fits in real workflows
Trying to conceive rather than already pregnant? The Ovulation Calculator estimates your fertile window from the same two inputs. The Period Calculator predicts your next six periods, which is also the fastest way to notice a missed one.
During pregnancy, weight and nutrition questions come up constantly — the BMI Calculator and Calorie Calculator can provide reference numbers to discuss with your doctor (standard BMI categories don't apply during pregnancy).
→ Ovulation Calculator · Period Calculator · BMI Calculator · Calorie Calculator
Frequently Asked Questions
How many weeks pregnant am I?
Count from the first day of your last menstrual period (LMP) to today: every full 7 days is one week. This calculator does it for you and shows the result as "X weeks Y days", adjusted for your cycle length. Note that pregnancy weeks are counted from the LMP, not from conception — so "6 weeks pregnant" means about 4 weeks after conception.
How is the due date calculated?
By Naegele's rule: due date = first day of last period + 280 days (40 weeks). If your cycle is longer or shorter than 28 days, the estimate shifts by the difference — e.g. with a 32-day cycle the due date moves 4 days later, because ovulation happens later in a longer cycle.
What if I know my conception date instead of my LMP?
Add 266 days (38 weeks) to the conception date to get the due date. Equivalently, subtract 14 days from the conception date to get a "virtual LMP" and enter that here with a 28-day cycle. If you conceived through IVF, your clinic will date the pregnancy from the transfer date — use their number.
My ultrasound gives a different due date. Which one is right?
The ultrasound. An early ultrasound (before 14 weeks) measures the embryo directly and dates the pregnancy to within about 5–7 days, which is more accurate than any date-based formula — especially if your cycles are irregular or you're not sure of your LMP. When the ultrasound date differs from the LMP date by more than about a week, doctors use the ultrasound date.
Is this a medical diagnosis? Is my data stored?
No and no. This is a statistical estimate based on average cycles — it cannot confirm a pregnancy, assess its health, or replace prenatal care. Individual variation is large, and irregular cycles make the estimate less reliable. Confirm everything with your doctor. The dates you enter are processed in your browser with JavaScript and are never sent to a server, stored or logged.