Pregnancy Month Calculator

Converting Pregnancy Weeks to Months: Why the Math Never Adds Up

You just found out you are 18 weeks pregnant, and someone asks the inevitable question: "So how many months is that?" You pause, do some mental math, and realize you have no idea what to say. Four months? Five months? The confusion is real, and you are not alone. Pregnant people across countless forums describe this exact frustration: the pregnancy week-to-month conversion simply does not work the way anyone expects it to. The problem is not your math skills. The problem is that pregnancy timing operates on a system that predates our modern calendar, and nobody bothered to reconcile the two.

The 40 Weeks vs 9 Months Paradox

Here is where the math breaks down for everyone. A common assumption is that one month equals four weeks, so 40 weeks should equal 10 months. But that would mean pregnancy lasts longer than the famous "nine months" everyone talks about. The actual calculation works differently. Most calendar months contain 30 or 31 days, not 28. When you divide 365 days by 12 months, each month averages about 30.4 days, or roughly 4.3 weeks. So 40 weeks multiplied by 7 days gives you 280 days total. Divide that by 30.4, and you get approximately 9.2 months. This is why doctors say pregnancy lasts "about 9 months" even though it is technically 40 weeks. February is the only month with exactly four weeks, which is why the "10 months" claim falls apart.

Adding another layer of confusion: the first two weeks of your "pregnancy" occur before you even conceive. Medical professionals count from the first day of your last menstrual period (LMP), not from conception. This means when you get a positive test around week 4, you have actually only been pregnant for about two weeks. If you subtract those pre-conception weeks, you are really only carrying your baby for about 38 weeks, or roughly 8.8 calendar months. No wonder the numbers feel wrong.

Lunar Months: The Hidden Third System

Before modern obstetrics, pregnancy was tracked using lunar months. A lunar month lasts approximately 29.5 days, which is close to a typical menstrual cycle length. Under this system, pregnancy spans 10 lunar months. Many traditional cultures still reference this timing, which is why some older relatives might insist pregnancy is "actually 10 months." They are not wrong, just using a different measurement system. Modern medicine switched to weeks because it provides more precision for tracking fetal development, but the lunar month concept persists in cultural understanding and adds to the general confusion when pregnant people try to answer the "how many months" question.

Why Every Source Shows Different Conversions

Search online for a weeks-to-months chart and you will find contradicting information everywhere. One app says 20 weeks equals 5 months, another says 4.5 months, and your pregnancy book lists something different entirely. This happens because there is no standardized conversion method. Some sources use strict 4-week months, others use calendar month calculations, and some create their own hybrid systems. Medical professionals avoid this problem by speaking exclusively in weeks, which is why your OB will never tell you how many months pregnant you are. The pregnancy week calculator on this site uses the medically accurate week system your provider relies on.

The inconsistency creates real frustration for pregnant people trying to communicate their progress to friends and family. Forums are full of posts from people who gave up trying to convert weeks to months and now just answer with their week number, often to confused looks from non-pregnant relatives. Some have started carrying a conversion cheat sheet on their phones just to handle the question. Others report simply rounding to the nearest month and accepting that precision is impossible in casual conversation.

A Practical Month-by-Month Breakdown

While no conversion is perfect, here is a commonly accepted framework that aligns with how most healthcare providers think about trimesters. Month 1 covers weeks 1 through 4. Month 2 spans weeks 5 through 8. Month 3 includes weeks 9 through 13, completing the first trimester. The second trimester begins with month 4 at weeks 14 through 17, continues through month 5 at weeks 18 through 22, and finishes month 6 at weeks 23 through 26. The third trimester starts with month 7 covering weeks 27 through 30, month 8 spanning weeks 31 through 35, and finally month 9 from weeks 36 through 40. Note that month 3 and some later months contain five weeks, which accounts for the extra days in calendar months.

How to Answer "How Far Along Are You?"

For casual conversations with grandparents, coworkers, or strangers at the grocery store, give them the month approximation since that is what they understand. Saying "about six months" is more relatable than "24 weeks and 3 days." For medical appointments, always use your exact weeks and days. This precision matters because significant developmental milestones happen within single weeks. The viability threshold at 24 weeks, the anatomy scan window, and testing timelines all depend on accurate week calculations. If you need to find your exact gestational age for medical purposes, the gestational age calculator provides the precise dating your healthcare team needs.

For calculating your expected delivery window, the due date calculator uses the same 280-day standard your provider applies. Remember that only about 5% of babies arrive on their exact due date, and full-term delivery anywhere from 37 to 42 weeks is considered normal. Whether you prefer tracking by weeks, months, or trimesters, what matters most is understanding where you are in the process and what developmental stage your baby has reached.

The Trimester Divide: Another Source of Confusion

Even trimester boundaries create debate. Most sources agree the first trimester ends after week 12, but the exact division between second and third trimester varies. Some define it at week 26, others at week 27, and the American College of Obstetricians and Gynecologists sets it at 28 weeks. This matters for preterm labor classifications and insurance coverage in some cases. For practical purposes, know that you are solidly in your third trimester by week 28, entering the home stretch where your baby primarily focuses on gaining weight and developing lung maturity. The middle trimester, often called the "honeymoon period," typically spans months 4 through 6 when many pregnant people report feeling their best.