Pregnancy Weeks to Months Converter
Convert pregnancy weeks to months, trimester, and percentage complete. Calculate weeks/days remaining to your 40-week due date instantly.
- American College of Obstetricians and Gynecologists data · June 2026
- Edited by Martín Rodríguez
- Private — runs on your device
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When to use this calculator
- Quickly translate your OB's week-based milestones into calendar months
- Determine which trimester you're currently in for symptom or nutrition guidance
- Calculate how many weeks and days remain until your estimated due date
- Track percentage of pregnancy completed for weekly progress updates
- Explain your stage of pregnancy to family or employers in months
- Plan maternity leave or baby shower timing based on trimester boundaries
Pregnancy Weeks to Months & Trimester Reference
| Weeks | Pregnancy Month | Trimester | Days Elapsed (start–end) |
|---|---|---|---|
| 1 – 4w6d | Month 1 | 1st Trimester | 1 – 35 |
| 5 – 8w6d | Month 2 | 1st Trimester | 29 – 63 |
| 9 – 13w6d | Month 3 | 1st Trimester | 57 – 97 |
| 14 – 17w6d | Month 4 | 2nd Trimester | 92 – 125 |
| 18 – 21w6d | Month 5 | 2nd Trimester | 120 – 153 |
| 22 – 26w6d | Month 6 | 2nd Trimester | 148 – 188 |
| 27 – 30w6d | Month 7 | 3rd Trimester | 183 – 216 |
| 31 – 35w6d | Month 8 | 3rd Trimester | 211 – 251 |
| 36 – 40w | Month 9 | 3rd Trimester | 245 – 280 |
Fuente: American College of Obstetricians and Gynecologists – ACOG (2023). Trimester boundaries per ACOG definitions; pregnancy months standardized at 28 days over a 40-week (280-day) full-term gestational period from LMP.
How it works
What is pregnancy week to month conversion?
Pregnancy is measured in weeks, but most people think in months. A full-term pregnancy spans 40 weeks, equivalent to approximately 9 months and 7 days. Converting weeks to months helps expectant parents align medical timelines with intuitive understanding, track trimester progression, and calculate remaining time until the 40-week due date accurately.
How It Works
Pregnancy duration is standardized at 40 weeks (280 days) from the first day of your last menstrual period (LMP). Weeks and months don't divide evenly, which is why a "9-month" pregnancy actually spans 9 months and 7 days on the calendar.
Formula
Total days elapsed = (weeks × 7) + extra days
Month equivalent = ceil(total days elapsed / (280 / 10))
→ using 28-day pregnancy months (280 ÷ 10 = 28)
Alternatively, approximate calendar months:
Month = floor(total days / 30.4375) + 1
Percent complete = (total days / 280) × 100
Days remaining = 280 − total days
Weeks remaining = floor(days remaining / 7)
Remainder days = days remaining mod 7Practical month mapping used by most midwives and OBs (based on 4-week "pregnancy months"):
| Weeks | Pregnancy Month | Trimester |
|---|---|---|
| 1 – 4w6d | Month 1 | 1st Trimester |
| 5 – 8w6d | Month 2 | 1st Trimester |
| 9 – 13w6d | Month 3 | 1st Trimester |
| 14 – 17w6d | Month 4 | 2nd Trimester |
| 18 – 21w6d | Month 5 | 2nd Trimester |
| 22 – 26w6d | Month 6 | 2nd Trimester |
| 27 – 30w6d | Month 7 | 3rd Trimester |
| 31 – 35w6d | Month 8 | 3rd Trimester |
| 36 – 40w | Month 9 | 3rd Trimester |
Trimester Boundaries
These boundaries follow ACOG (American College of Obstetricians and Gynecologists) definitions.
Worked Example
You are 20 weeks and 3 days pregnant.
1. Total days = (20 × 7) + 3 = 143 days
2. Percent complete = 143 / 280 × 100 = 51.1%
3. Days remaining = 280 − 143 = 137 days → 19 weeks 4 days
4. Month: Week 20 falls in Month 5 (weeks 18–21w6d)
5. Trimester: Week 20 is in the 2nd Trimester (weeks 14–27w6d)
Limitations
Example: 20 weeks and 3 days pregnant
Frequently asked questions
Why is pregnancy measured in weeks instead of months?
How many weeks are in each month of pregnancy?
When does each trimester start and end?
What does 'full term' mean?
I'm at 4 weeks pregnant — does that mean I just conceived?
Can I enter week 41 or 42?
Why does my app say I'm in month 5 but this calculator says month 4?
What is the percentage complete used for?
How accurate is the weeks-to-months conversion?
Sources & references
- ACOG – How Your Due Date Is Calculated — American College of Obstetricians and Gynecologists (2023)
- CDC – Preterm Birth — Centers for Disease Control and Prevention (2024)
- NIH MedlinePlus – Fetal Development — U.S. National Library of Medicine (2024)
- ACOG Committee Opinion 764 – Medically Indicated Late-Preterm and Early-Term Deliveries — American College of Obstetricians and Gynecologists (2019)
Methodology & trust
familia calculator reviewed by the Hacé Cuentas editorial team, checked against ACOG – How Your Due Date Is Calculated, following our editorial policy and methodology.
Last reviewed: June 2026. Parameters are verified periodically against the cited sources.
Calculations run 100% in your browser. We do not store or transmit your data.
Indicative results. For critical decisions, consult a professional.
Rodríguez, M. (2026). Pregnancy Weeks to Months Converter. Hacé Cuentas. https://hacecuentas.com/en/pregnancy-weeks-to-months-converter
Content licensed under CC-BY 4.0 — reuse it citing the source with a link to Hacé Cuentas.