Calculating Pregnancy Weeks to Months

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Pregnancy Weeks to Months Calculator

Accurately track your pregnancy progress and trimesters

How to Calculate Pregnancy Weeks to Months

A common source of confusion for many expecting parents is the conversion between weeks and months. While a standard calendar month is roughly 4.3 weeks long, medical professionals track pregnancy based on a 40-week cycle. This is because it is easier to track fetal development milestones on a weekly basis.

The Trimester Breakdown

Pregnancy is divided into three distinct phases called trimesters. Understanding where you fall in these phases helps you prepare for the physical and hormonal changes ahead:

  • First Trimester: Weeks 1 through 13 (Months 1-3)
  • Second Trimester: Weeks 14 through 27 (Months 4-6)
  • Third Trimester: Weeks 28 through 40+ (Months 7-9+)

Pregnancy Conversion Chart

Month Weeks
Month 1Weeks 1, 2, 3, 4
Month 2Weeks 5, 6, 7, 8
Month 3Weeks 9, 10, 11, 12, 13
Month 4Weeks 14, 15, 16, 17
Month 5Weeks 18, 19, 20, 21, 22
Month 6Weeks 23, 24, 25, 26, 27
Month 7Weeks 28, 29, 30, 31
Month 8Weeks 32, 33, 34, 35
Month 9Weeks 36, 37, 38, 39, 40

Why use a Pregnancy Weeks Calculator?

Calculating your pregnancy in months can be tricky because most months aren't exactly four weeks long. If you assume every month is 4 weeks, a 40-week pregnancy would seem like 10 months. Our calculator uses the standard medical grouping to give you the most accurate answer used by OB-GYNs and midwives.

Note: Pregnancy is measured from the first day of your last menstrual period (LMP). This means in weeks 1 and 2, you aren't technically pregnant yet, but these weeks are included in the 40-week count.

function calculatePregnancy() { var weeksVal = document.getElementById('pregWeeks').value; var resultDiv = document.getElementById('pregResult'); var monthOut = document.getElementById('monthOutput'); var triOut = document.getElementById('trimesterOutput'); var infoOut = document.getElementById('infoOutput'); if (weeksVal === "" || weeksVal = 1 && weeks = 5 && weeks = 9 && weeks = 14 && weeks = 18 && weeks = 23 && weeks = 28 && weeks = 32 && weeks = 36 && weeks 40) { month = "Month 10 (Post-Term)"; trimester = "Third Trimester"; info = "You've reached your due date! Most doctors won't var pregnancy go past 42 weeks."; } monthOut.innerHTML = "You are in " + month; triOut.innerHTML = "Trimester: " + trimester; infoOut.innerHTML = "Stage Info: " + info; resultDiv.style.display = "block"; }

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