Official pages count pregnancy in weeks, and none we read prints a weeks-to-months table. This page converts weeks to months with its method shown, compares how NICHD, the Office on Women’s Health and the NHS divide the trimesters, and lists what to ask your doctor about your dates.
Pregnancy usually lasts about 40 weeks, or just over 9 months, counted from the last menstrual period to delivery, according to the US National Institute of Child Health and Human Development (NICHD, last reviewed 29 May 2024). Official pages count in weeks, so the months in the table below are our own arithmetic. Your doctor’s dating, which may use a scan, governs.
This page only converts units and compares sources. It does not date your pregnancy or advise you.
In short:
- No page we read prints a weeks-to-months table, so the months below are derived: at 30.44 days a month, 40 weeks (280 days) is 9 months and about 6 days.
- Trimester edges differ from page to page, and a week on a boundary is a question for your doctor.
Last checked: 31 October 2026, on the pages listed under Sources. Pages change, so confirm with your doctor.
How many weeks is a pregnancy, and from which day do the weeks count?
The US Office on Women’s Health (updated 26 September 2025) says pregnancy lasts about 40 weeks, counting from the first day of your last normal period, and MedlinePlus (review date 18 August 2025) measures gestational age the same way. The NHS (page reviewed 21 August 2024) says it normally lasts 37 to 42 weeks from the first day of your last period, and that its 12-week scan can more accurately estimate how many weeks pregnant you are. So the count you are given can be revised: ask your doctor which date it is counted from.
Pregnancy weeks to months: the conversion table
Method: whole weeks counted from the first day of the last period, each placed in the month in which its first day falls, taking a month as 30.44 days (365.25 ÷ 12). This is arithmetic, not a published table. The one month figure from a source is NICHD’s: 40 weeks is just over 9 months.
| Month of pregnancy (derived) | Whole weeks in that month | Trimester on the NICHD and Office on Women’s Health pages |
|---|---|---|
| 1st | 0 to 4 | First (weeks 1 to 12) |
| 2nd | 5 to 8 | First |
| 3rd | 9 to 13 | First up to week 12; second from week 13 |
| 4th | 14 to 17 | Second (weeks 13 to 28) |
| 5th | 18 to 21 | Second |
| 6th | 22 to 26 | Second |
| 7th | 27 to 30 | Second up to week 28; third from week 29 |
| 8th | 31 to 34 | Third (weeks 29 to 40) |
| 9th | 35 to 39 | Third |
| At 40 weeks | 280 days: 9 months and about 6 days (NICHD: just over 9 months) | Third (to week 40) |
Months: our arithmetic. Trimesters: the week numbers printed by NICHD (last reviewed 29 May 2024) and the Office on Women’s Health (updated 26 September 2025), read 31 October 2026. Counting four-week months gives different boundaries, and 40 weeks would be ten of them. Your doctor’s dating governs.
My report says weeks and days: which month is that?
Use the weeks to find the row: 20 weeks and 2 days falls in the 18 to 21 row, the fifth month on this arithmetic. Each week is placed by the day it begins, so the months are approximate by a few days at a boundary.
Where do the first, second and third trimesters start and end?
| Source | First trimester | Second trimester | Third trimester |
|---|---|---|---|
| NICHD (last reviewed 29 May 2024) | Weeks 1 to 12 | Weeks 13 to 28 | Weeks 29 to 40 |
| US Office on Women’s Health (updated 26 September 2025) | Weeks 1 to 12 | Weeks 13 to 28 | Weeks 29 to 40 |
| NHS week-by-week guide (no date shown) | Weeks 4 to 12 | Weeks 13 to 27 | Weeks 28 to 41 |
The pages we read do not say whether their edges are completed weeks or the week in progress, so a week on a boundary (12 or 13, 27 or 28, 28 or 29) is a question for your doctor.
What should I ask my doctor about my dates?
- How many weeks and days am I today, and from which date is that counted?
- Has my dating changed since a scan, and which date should I use now?
- What is my due date, and how was it worked out?
- Can you write the week next to each date in my notes?
In Egypt: where can I see pregnancy weeks and months set out?
The Ministry of Health and Population’s Maternal and Child Health page (undated) lists medical and laboratory examination at least five times during pregnancy (read 31 October 2026). The State Information Service’s item dated 3 March 2026 (the page also shows 5 March 2026) describes the comprehensive medical examination of the Presidential Initiative for Maternal and Fetal Health, and the vitamins and nutrients it provides, as free.
Neither names a week or month of pregnancy, and we found no Egyptian official weeks-to-months table when we searched on 31 October 2026, which does not prove none exists. Visits and scans by week are in our pregnancy follow-up guide, which sets them out from the World Health Organization’s schedule.
Which signs mean going to the emergency department now?
The US Centers for Disease Control and Prevention (CDC) says to seek medical care immediately if you have any sign on its urgent maternal warning signs list (page dated 15 May 2024, read 31 October 2026). Our pregnancy follow-up guide sets out the signs on that list that apply during pregnancy. If you have any of those signs, go to the nearest emergency department or call emergency services now. Do not wait for the next appointment.
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Medigize is a navigation tool. It does not provide antenatal care, date a pregnancy or interpret results. We are building a record that moves with the patient, shared only with their consent. If you would like to follow that work, you can register your interest.
Frequently asked questions
How many weeks is a pregnancy?
NICHD (last reviewed 29 May 2024) says usually about 40 weeks, or just over 9 months, from the last menstrual period; the NHS (page reviewed 21 August 2024) says normally 37 to 42 weeks. Your doctor’s dating governs.
How many months is 40 weeks of pregnancy?
NICHD (last reviewed 29 May 2024) says just over 9 months. By our arithmetic, 280 days is 9 months and about 6 days.
If I am 20 weeks pregnant, which month is that?
The fifth month in our table (the 18 to 21 row). That is derived arithmetic: no source we read prints a weeks-to-months table.
Medigize is a non-diagnostic navigation tool. This is general information, not medical advice. In an emergency, call an ambulance or go to the nearest emergency department.
Sources
- Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), US National Institutes of Health: About Pregnancy, last reviewed 29 May 2024, read 31 October 2026
- US Office on Women’s Health (womenshealth.gov): Stages of pregnancy, updated 26 September 2025, read 31 October 2026
- MedlinePlus Medical Encyclopedia (US National Library of Medicine): Fetal development, review date 18 August 2025, read 31 October 2026
- NHS: Pregnancy due date calculator, page reviewed 21 August 2024 (next review due 21 August 2027), read 31 October 2026
- NHS: Week-by-week guide to pregnancy (no date shown), read 31 October 2026
- Egyptian Ministry of Health and Population, Primary Health Care Sector: Maternal and Child Health (in Arabic, undated page), read 31 October 2026
- State Information Service: The Presidential Initiative for Maternal and Fetal Health screens 3.75 million pregnant women free of charge (in Arabic, 3 March 2026; the page also shows 5 March 2026), read 31 October 2026
- US Centers for Disease Control and Prevention: Urgent maternal warning signs and symptoms (page dated 15 May 2024), read 31 October 2026
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