Reference chart
Pregnancy Weeks to Months Chart
By &Pixels, the studio that builds MyCalculator.to. This explains how the counting works. Your midwife or doctor is the authority on your own pregnancy.
Weeks, months and trimesters side by side, and why 40 weeks is not nine months.
Why is pregnancy 40 weeks?
Two reasons, and both are about how the counting works rather than about biology. Forty weeks is 9.2 calendar months, because a month averages about 4.3 weeks rather than 4. And the count begins at the first day of the last menstrual period, roughly two weeks before conception.
That convention exists because the date of a period can be known and the date of conception usually cannot. It does mean the first fortnight of a forty week count happens before there is a pregnancy, which is the part nobody mentions and which makes the arithmetic feel wrong.
Weeks, months and trimesters
| Weeks | Month | Trimester |
|---|---|---|
| 1 to 4 | Month 1 | First |
| 5 to 8 | Month 2 | First |
| 9 to 13 | Month 3 | First |
| 14 to 17 | Month 4 | Second |
| 18 to 22 | Month 5 | Second |
| 23 to 27 | Month 6 | Second |
| 28 to 31 | Month 7 | Third |
| 32 to 35 | Month 8 | Third |
| 36 to 40 | Month 9 | Third |
The groupings are conventional rather than exact, and different sources draw the month boundaries in slightly different places. Clinicians work in weeks and days, written as 32+4 for thirty-two weeks and four days, and that notation is always the more precise one.
The calculators behind this
What the terms mean
| Term | Weeks | Note |
|---|---|---|
| Preterm | before 37 weeks | earlier than full development typically completes |
| Early term | 37 weeks to 38 weeks 6 days | a distinct category since 2013 |
| Full term | 39 weeks to 40 weeks 6 days | the outcomes are best in this window |
| Late term | 41 weeks to 41 weeks 6 days | still within normal |
| Post term | 42 weeks and beyond | usually monitored more closely |
How the weeks are counted
- Start from the first day of the last period. Not from conception. This is the convention every chart and scan uses.
- Count in weeks, not months. Clinicians work in weeks and days, written as 32+4 for thirty-two weeks and four days.
- Add 280 days for the estimated due date. Forty weeks from that first day. This is Naegele’s rule, which is what most calculators use.
- Expect a scan to adjust it. An early ultrasound dates a pregnancy more accurately and the date is often revised.
- Treat the date as an estimate. Very few babies arrive on it. The window either side is normal, not late or early.
For a date rather than a table, the due date calculator works from a last period or a conception date, and the gestational age calculator gives the weeks-and-days figure for any date, which is the notation your notes will use.
A due date is an estimate, and very few babies arrive on one. Arriving in the weeks either side is normal rather than early or late. Anything you are wondering about is a question for your midwife or doctor, who knows your pregnancy in a way a chart cannot.
Formula & Methodology
Did you know? Naegele's rule, the 280 day calculation behind most due dates, was published in the early nineteenth century and is still the standard starting point. An early ultrasound now dates a pregnancy more accurately, which is why the estimate is often revised after the first scan.
Sources
- American College of Obstetricians and Gynecologists, Committee Opinion on the definition of term pregnancy
- ACOG, Methods for Estimating the Due Date
- NHS guidance on pregnancy dating and antenatal care