Pregnancy Weeks to Months Converter
Pregnancy is measured in weeks, but most people think in months — and the two don't line up neatly. This converter takes your current week and day of pregnancy and tells you the equivalent month, your trimester, how many weeks and days remain until your 40-week full-term date, and how far along you are as a percentage. Full term is defined as 40 weeks (280 days), roughly 9 months and 7 days.
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
How it works
2 min readWhat 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
Frequently asked questions
Why is pregnancy measured in weeks instead of months?
Weeks provide more precise tracking for fetal development milestones, ultrasound measurements, and clinical decision-making. A one-week difference (e.g., 36 vs. 37 weeks) can have significant clinical implications for neonatal outcomes that a month-based system would obscure.
How many weeks are in each month of pregnancy?
Pregnancy is divided into 10 'pregnancy months' of 4 weeks (28 days) each, totaling 280 days. This differs from calendar months, which average 30.4 days. That's why a '9-month' pregnancy is actually 9 months and about 7 days.
When does each trimester start and end?
Per ACOG: 1st trimester = weeks 1–13 (through day 6 of week 13); 2nd trimester = weeks 14–27 (through day 6 of week 27); 3rd trimester = weeks 28–40. Some sources place the 2nd/3rd boundary at week 28, which this calculator uses.
What does 'full term' mean?
ACOG defines full term as 39 weeks 0 days through 40 weeks 6 days. 'Early term' is 37–38w6d. The standard estimated due date (EDD) is set at exactly 40 weeks (280 days) from the LMP. Births before 37 weeks are considered preterm.
I'm at 4 weeks pregnant — does that mean I just conceived?
Not exactly. Gestational age counts from the first day of your last menstrual period (LMP), not conception. Conception typically occurs around week 2. So at 4 weeks gestational age, you are approximately 2 weeks past conception.
Can I enter week 41 or 42?
Yes. Pregnancies can extend past 40 weeks. Weeks 41 and 42 are shown as post-term. The calculator will show a negative days-remaining value and a percentage over 100%, indicating you have passed the standard EDD.
Why does my app say I'm in month 5 but this calculator says month 4?
Different apps use different month-mapping conventions. Some map by 4-week blocks (this calculator's default), some use calendar months, and some count months differently. The obstetric standard is 4-week pregnancy months, which is what this tool uses.
What is the percentage complete used for?
It's a simple progress metric: days elapsed ÷ 280 × 100. At 20 weeks exactly you are 50% complete. It's useful for communicating progress visually but has no clinical significance on its own.
How accurate is the weeks-to-months conversion?
The conversion is mathematically precise given the standard 280-day / 40-week definition. However, individual pregnancies vary — ultrasound dating may revise your gestational age, and your actual due date could shift by 1–2 weeks based on early ultrasound findings.