Weeks to Months Methodology
This page documents the convention, the exact formula, and the sources behind the DueDateLab Weeks to Months calculator and its week-by-week pages. It exists for transparency: anyone can verify how a week number becomes a month number. All conversion is arithmetic and runs in your browser.
Pregnancy is dated in weeks from the first day of the last menstrual period (ACOG, NHS). There is no single clinical definition of a pregnancy “month.” DueDateLab uses a transparent four-week convention: 1 month = 4 weeks = 28 days, so pregnancy months = weeks ÷ 4. A second column compares against the average calendar month of 30.4375 days (365.25 ÷ 12), so the same week reads slightly smaller in calendar months. Both numbers are correct; they describe the same span with different month definitions.
Open the Weeks to Months calculator →
Summary
The calculator turns a gestational week into two month figures. The headline figure uses a four-week pregnancy month (weeks ÷ 4), which produces clean, predictable numbers and matches the common “10 months of pregnancy” framing (40 weeks ÷ 4 = 10). A second figure converts the same elapsed time into average calendar months for readers who think in ordinary months. Neither figure is a clinical measurement; gestational age itself is always the week count from the last menstrual period.
Why pregnancy is dated in weeks, not months
Clinical pregnancy dating counts completed weeks from the first day of the last menstrual period (LMP), across a roughly 40-week span, with ultrasound used to confirm or revise the estimate. ACOG Committee Opinion 700 sets out the standard methods for estimating the due date, and the NHS due-date guidance uses the same week-based dating. Months are only a friendlier summary layered on top. Because calendar months vary between 28 and 31 days, there is no exact, agreed number of days in a “pregnancy month,” which is why different sites give different month answers for the same week.
Formula 1, Pregnancy months (four-week convention)
Given a gestational week:
Pregnancy months = weeks ÷ 4
One pregnancy month is defined as exactly four weeks, or 28 days. This is the headline figure on every week page. It is the convention behind the familiar statement that pregnancy lasts about ten months (40 weeks ÷ 4 = 10), even though people often say “nine months” using calendar months. The four-week month is an arithmetic convention chosen for clarity, not a clinical unit.
Formula 2, Calendar-month comparison
Given a gestational week:
Calendar months = (weeks × 7) ÷ 30.4375
The comparison column converts the elapsed days (weeks × 7) into average calendar months, using 30.4375 days per month (365.25 ÷ 12, which accounts for leap years). Because the average calendar month is longer than 28 days, the same week yields a slightly smaller number here than under the four-week convention. This is the figure that produces the common “nine months” total (280 days ÷ 30.4375 ≈ 9.2).
Worked example, 17 weeks
At 17 weeks:
- Days elapsed: 17 × 7 = 119 days.
- Pregnancy months (four-week): 17 ÷ 4 = 4.25 months.
- Calendar months: 119 ÷ 30.4375 ≈ 3.9 months.
Both describe the same 119 days. The four-week figure is larger because its month is shorter.
Reference points
Selected milestones, using the two conventions above:
| Week | Pregnancy months (÷4) | Calendar months | Days |
|---|---|---|---|
| 4 | 1.0 | ~0.9 | 28 |
| 8 | 2.0 | ~1.8 | 56 |
| 13 | 3.25 | ~3.0 | 91 |
| 27 | 6.75 | ~6.2 | 189 |
| 40 | 10.0 | ~9.2 | 280 |
The full week 1 to week 42 chart lives on the Weeks to Months calculator, with a dedicated page for each week.
Why month answers differ across sites
Two sites can both be right and still disagree, because they pick different definitions of a month. A site using the four-week convention will say 40 weeks is 10 months; a site using calendar months will say about 9. Some sites also round differently or count from conception (about two weeks later) rather than from LMP. DueDateLab shows both the four-week figure and the calendar-month figure side by side so the difference is visible rather than hidden, and always anchors the underlying date in weeks from LMP.
Assumptions and limitations
- The conversion assumes standard LMP-based dating. If your gestational age has been revised by an ultrasound, use the revised week number.
- “Months” here is a readability aid, not a clinical measure. Care decisions use the week number and, where relevant, the estimated due date.
- The four-week month (28 days) and the average calendar month (30.4375 days) are arithmetic conventions, not measured biological units.
- For a calendar date for the birth, use the Due Date Calculator rather than a month figure.
Sources
- American College of Obstetricians and Gynecologists. Committee Opinion 700: Methods for Estimating the Due Date. ACOG. Cited for week-based gestational dating from the last menstrual period and the standard 40-week span.
- NHS. Pregnancy due date calculator. NHS UK. Cited for week-based pregnancy dating in routine care.
Authorship
The DueDateLab Editorial team writes and maintains this page. Editorial is not a clinical practice. The week-based dating claims are bound to the ACOG and NHS sources above; the month figures are transparent arithmetic conversions, clearly labeled as such. DueDateLab is published from Belgium.
Conflicts of interest. DueDateLab is supported by advertising and may use affiliate links. Advertising and affiliate relationships do not influence the methodology or sources on this page.
Last updated July 12, 2026.