How pregnancy weeks are counted
Doctors measure pregnancy in completed weeks and days counted from the first day of your last period. Day 1 of your period is “0 weeks 0 days”. Seven days later you are 1 week 0 days, and so on until 40 weeks 0 days – your due date.
Counting in weeks rather than months is deliberate. A baby’s development changes a lot within a single month, so screening tests, scans and vaccines are timed to specific weeks. A calendar month is too blunt a measure – “4 months pregnant” could mean anything from 13 to 18 weeks.
“12 weeks” vs “12 weeks 3 days”
On scan reports and in hospital notes you’ll see gestational age written as 12w 3d or 12+3. Both mean 12 full weeks plus 3 days. Days matter at certain points: the NT scan must be done before 13w 6d, for example, and some blood tests are only valid within a narrow window.
Being “in week 13” vs “13 weeks pregnant”
This trips up many people. Once you have completed 12 weeks, you are in your 13th week – just as a baby is in their second year after their first birthday. Our calculator shows both: “12 weeks 3 days pregnant” and “pregnancy week 13”. Our week-by-week guide is organised by completed weeks, so 12 weeks 3 days takes you to the 12 weeks pregnant page.
How many days pregnant are you? The calculator also shows your total days – handy for IVF clinics and for comparing with scan reports.
Weeks at a glance
| Weeks | Trimester | What’s typically happening |
|---|---|---|
| 0–4 | 1st | Ovulation, conception, implantation; positive test around week 4 |
| 5–8 | 1st | Heartbeat visible on scan, nausea often begins |
| 9–13 | 1st | Embryo becomes a fetus; NT scan at 11–13+6 weeks |
| 14–20 | 2nd | Energy returns, bump shows, first movements (quickening) |
| 18–22 | 2nd | Detailed anomaly (Level II) scan |
| 24–28 | 2nd | Glucose tolerance test; baby reaches viability milestone |
| 28–36 | 3rd | Rapid weight gain, growth scan, kick counting |
| 37–42 | 3rd | Term – labour can begin any day |
Need your due date too? It’s in the results above, or use the dedicated due date calculator.
Frequently asked questions
How many weeks pregnant am I if my period was 2 months ago?
Roughly 8–9 weeks. Two calendar months is about 61 days, which is 8 weeks 5 days. For an exact answer enter the first day of that period in the calculator.
When does week 1 of pregnancy start?
Week 1 begins on the first day of your last period. Conception usually happens around the end of week 2, and a pregnancy test typically becomes positive in week 4–5.
How many weeks is a full-term pregnancy?
A full-term pregnancy is 39 weeks 0 days to 40 weeks 6 days. Babies born from 37 weeks are considered ‘early term’ and are generally well developed.
Why does my scan say a different week than the calculator?
Your LMP-based date assumes ovulation on a particular day. An early scan measures the baby directly, so it can differ by several days. If your doctor has re-dated your pregnancy, choose the ‘Due date’ or ‘Ultrasound’ tab and enter those details.
How many weeks is 9 months pregnant?
The ninth month is roughly weeks 36 to 40. Because months are uneven, 40 weeks actually works out to about 9 months and 1 week from your last period.
Medical disclaimer: The results and information on this page are estimates for general education only. They are not a diagnosis and do not replace advice from your gynaecologist, obstetrician or midwife. If you have pain, bleeding, reduced baby movements or feel unwell, contact your doctor or call 108 immediately. Read the full disclaimer.
Sources & further reading
- American College of Obstetricians and Gynecologists (ACOG). Committee Opinion No. 700: Methods for Estimating the Due Date. acog.org
- World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience (2016). who.int
- Ministry of Health & Family Welfare, Government of India. Maternal health guidelines, PMSMA and Anemia Mukt Bharat. nhm.gov.in