The moment you see a positive test, two questions follow: when is the baby due, and how far along am I? Both have standard answers, but the counting conventions are confusing at first. Pregnancy is measured in weeks and days from the first day of your last menstrual period (LMP), not from conception, and “40 weeks” does not translate neatly into nine months. This guide explains the rules. You can do the arithmetic instantly with our pregnancy due date calculator.
This guide is general information, not medical advice. Your midwife, GP or obstetrician will confirm dates and care for your pregnancy.
Why counting starts before conception
Most people do not know exactly when they conceived, but many know when their last period started. So doctors count gestational age from the first day of the LMP. In a typical 28-day cycle, ovulation and conception happen about two weeks later. That means:
- At conception you are already about 2 weeks pregnant.
- A positive test around the date of a missed period usually means you are about 4 weeks pregnant.
- Embryonic or fetal age (from conception) is about two weeks less than gestational age.
Calculating the due date
Naegele’s rule
The estimated due date (EDD) is LMP + 280 days (40 weeks). The classic shortcut:
First day of LMP + 1 year − 3 months + 7 days
Example: LMP on 1 January 2026 → add 1 year (1 January 2027) → subtract 3 months (1 October 2026) → add 7 days → 8 October 2026. Counting 280 days directly from 1 January 2026 gives the same date.
Adjusting for longer or shorter cycles
Naegele’s rule assumes a 28-day cycle. If your cycle is regularly longer, ovulation happens later, so the due date moves later by the same number of days. For a 32-day cycle, add 4 days; for a 25-day cycle, subtract 3 days.
Dating scan and IVF
- A first-trimester ultrasound measuring the baby’s crown–rump length is the most accurate way to date a pregnancy. If it differs significantly from the LMP date, the scan date is usually used.
- After IVF, the date is calculated from the embryo transfer: for a day-5 transfer, add 261 days to the transfer date (280 − 14 − 5).
Weeks, months and trimesters
Pregnancy is written as weeks plus days, for example 24+3 means 24 weeks and 3 days. “In week 24” means 23+0 to 23+6, which trips people up.
| Trimester | Weeks | Approximate months |
|---|---|---|
| First | 0 to 13+6 | Months 1–3 |
| Second | 14+0 to 27+6 | Months 4–6 |
| Third | 28+0 to birth | Months 7–9 |
Trimester boundaries vary slightly between sources (some end the first trimester at 12 weeks), but the NHS uses the split above.
Converting weeks to months
A calendar month is about 4.35 weeks, not 4. So:
| Weeks | Roughly |
|---|---|
| 13 | End of month 3 |
| 17–18 | End of month 4 |
| 22 | End of month 5 |
| 26–27 | End of month 6 |
| 31 | End of month 7 |
| 35 | End of month 8 |
| 40 | About 9 months and 1 week |
Key scans and appointments (NHS)
The NHS in England offers at least two ultrasound scans:
| Scan | Timing | Purpose |
|---|---|---|
| Dating scan | About 10 to 14 weeks (combined screening 11+2 to 14+1) | Confirm the due date, check heartbeat, number of babies; screening for Down’s, Edwards’ and Patau’s syndromes if you choose |
| Mid-pregnancy (anomaly) scan | 18+0 to 20+6 weeks | Detailed check of the baby’s development and placenta position |
Other milestones: the booking appointment ideally happens by 10 weeks, and screening blood tests are offered early. If you are in the US, scheduling varies by provider; many offer a first-trimester scan and a detailed anatomy scan at around 18–22 weeks.
What “term” means
The American College of Obstetricians and Gynecologists (ACOG) uses these labels:
| Category | Gestational age |
|---|---|
| Preterm | Before 37+0 weeks |
| Early term | 37+0 to 38+6 |
| Full term | 39+0 to 40+6 |
| Late term | 41+0 to 41+6 |
| Post-term | 42+0 and beyond |
Only a small proportion of babies arrive on the exact due date. If pregnancy continues past it, your maternity team will discuss monitoring and, typically from around 41 weeks, the option of induction.
Common mistakes
- Counting from conception. Apps and clinicians use LMP; a conception-based count will be two weeks behind.
- Treating “week 12” and “12 weeks” as the same. 12+0 is the start of week 13.
- Ignoring cycle length. A long, regular cycle shifts the due date.
- Using months for medical decisions. Clinical guidance is always in weeks and days.
- Expecting the due date to be exact. Plan for a window from about 37 to 42 weeks.
Checklist
- Note the first day of your last period.
- Add 280 days (adjusting for cycle length) to estimate the due date.
- Register with a midwife or doctor early; the booking appointment is ideally by 10 weeks.
- Expect the dating scan at about 10–14 weeks to confirm or adjust the date.
- Book the anomaly scan window, 18+0 to 20+6 weeks.
For counting days between any two dates, see our days between dates guide. If you want to check whether your weight is in a healthy range before pregnancy, our BMI guide explains the categories.