Pregnancy weeks to months: pregnancy is counted in weeks from the first day of your last period and lasts about 40 weeks, and because months run 4 to 5 weeks long, charts differ by a week or so, but 24 weeks is roughly 5.5 months, putting you in your 6th month.
Your provider says “24 weeks,” your mom asks “how many months is that?”, and three different apps give you three different answers. You are not bad at math. Weeks and months simply do not line up neatly, and there is no single official way to convert one into the other.
This guide gives you a clear pregnancy weeks to months chart, a lookup for every week from 1 to 42, the trimester boundaries as each source states them, how your due date is calculated and why providers stick to weeks.
Key Takeaways
Pregnancy is counted from the first day of your last menstrual period and lasts about 40 weeks (280 days).
Months are 4 to 5 weeks long, so weeks-to-months charts can differ by about a week. Go by weeks.
On our chart, month 1 is weeks 1 to 4, month 6 is weeks 23 to 27 and month 9 is weeks 36 to 40.
Sources split trimesters slightly differently, especially around week 13.
Full term is 39 weeks 0 days through 40 weeks 6 days, and only about 5% of babies arrive on the due date (Cleveland Clinic).
Table of Contents
Pregnancy Weeks to Months: The Short Answer
Quick Answer
Pregnancy is counted in weeks from the first day of your last menstrual period (LMP), and the Office on Women’s Health says it lasts about 40 weeks. Because calendar months run 4 to 5 weeks long, there is no single official weeks-to-months conversion, so charts can differ by a week or so. On the chart below, weeks 1 to 4 are month 1, weeks 23 to 27 are month 6, and weeks 36 to 40 are month 9.
If you only want a quick number, divide the weeks by about 4.3. That works because an average calendar month is about 30.4 days (365 days divided by 12), which is a little over 4 weeks. So 24 weeks is about 5.5 months completed, which means you are in your 6th month.
Pregnancy Weeks to Months Conversion Chart
Here is the convention we use on this page: month 1 = weeks 1 to 4, then roughly 4 to 5 weeks per month after that, finishing with month 9 at weeks 36 to 40. Other charts may group the weeks differently. For example, Cleveland Clinic’s fetal development guide splits the 40 weeks into ten blocks of exactly 4 weeks, so its month 9 is weeks 33 to 36 and it adds a month 10 for weeks 37 to 40.
Month
Weeks (this chart)
Trimester (Office on Women’s Health)
Weeks (Cleveland Clinic 4-week months)
Month 1
Weeks 1 to 4
First
Weeks 1 to 4
Month 2
Weeks 5 to 8
First
Weeks 5 to 8
Month 3
Weeks 9 to 13
First (weeks 9 to 12), second from week 13
Weeks 9 to 12
Month 4
Weeks 14 to 17
Second
Weeks 13 to 16
Month 5
Weeks 18 to 22
Second
Weeks 17 to 20
Month 6
Weeks 23 to 27
Second
Weeks 21 to 24
Month 7
Weeks 28 to 31
Second (week 28), third from week 29
Weeks 25 to 28
Month 8
Weeks 32 to 35
Third
Weeks 29 to 32
Month 9
Weeks 36 to 40
Third
Weeks 33 to 36
Month 10
Not used on this chart
Third
Weeks 37 to 40
Past the due date
Weeks 41 to 42
Late term, then post-term
Not listed
Pregnancy weeks to months. Week counts start from the first day of the last menstrual period. Month groupings are a convention, not a medical standard.
Neither grouping is wrong. They are two ways of slicing the same 40 weeks. What your provider writes in your chart is the week count, not the month, so if two charts disagree, go by weeks.
For a sense of how your body changes across these months, see our pregnant belly by month guide.
Week-by-Week Lookup: Weeks 1 to 42
Find your week in the left column. “Month” uses our chart above. “Trimester” uses the Office on Women’s Health boundaries (week 1 to 12, week 13 to 28, week 29 to 40). “Months completed” is simple math: weeks times 7 days, divided by 30.44 days, the average calendar month. It tells you how many full months have passed, which is why it is always a bit lower than the month you are in.
Week
Month (this chart)
Trimester (OWH)
Months completed (approx.)
Week 1
Month 1
First
0.2
Week 2
Month 1
First
0.5
Week 3
Month 1
First
0.7
Week 4
Month 1
First
0.9
Week 5
Month 2
First
1.1
Week 6
Month 2
First
1.4
Week 7
Month 2
First
1.6
Week 8
Month 2
First
1.8
Week 9
Month 3
First
2.1
Week 10
Month 3
First
2.3
Week 11
Month 3
First
2.5
Week 12
Month 3
First
2.8
Week 13
Month 3
Second
3.0
Week 14
Month 4
Second
3.2
Week 15
Month 4
Second
3.4
Week 16
Month 4
Second
3.7
Week 17
Month 4
Second
3.9
Week 18
Month 5
Second
4.1
Week 19
Month 5
Second
4.4
Week 20
Month 5
Second
4.6
Week 21
Month 5
Second
4.8
Week 22
Month 5
Second
5.1
Week 23
Month 6
Second
5.3
Week 24
Month 6
Second
5.5
Week 25
Month 6
Second
5.7
Week 26
Month 6
Second
6.0
Week 27
Month 6
Second
6.2
Week 28
Month 7
Second
6.4
Week 29
Month 7
Third
6.7
Week 30
Month 7
Third
6.9
Week 31
Month 7
Third
7.1
Week 32
Month 8
Third
7.4
Week 33
Month 8
Third
7.6
Week 34
Month 8
Third
7.8
Week 35
Month 8
Third
8.0
Week 36
Month 9
Third
8.3
Week 37
Month 9
Third
8.5
Week 38
Month 9
Third
8.7
Week 39
Month 9
Third
9.0
Week 40
Month 9
Third
9.2
Week 41
Past 40 weeks
Past the due date
9.4
Week 42
Past 40 weeks
Past the due date
9.7
Weeks 1 to 42 converted to months. Months completed = weeks x 7 / 30.44, rounded to one decimal.
Notice the gap between the two middle columns. At 24 weeks, about 5.5 months have passed, so you are living through your 6th month. Both answers are reasonable, and that difference explains most of the confusion online.
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How Many Months Is 12, 16, 20, 24, 28, 32 or 36 Weeks?
These are the weeks people search most. Each answer comes from the chart and lookup above, with the same caveat: charts can shift by a week.
Weeks pregnant
Month you are in (this chart)
Months completed
Cleveland Clinic 4-week months
12 weeks
Month 3
About 2.8
End of month 3
16 weeks
Month 4
About 3.7
End of month 4
20 weeks
Month 5
About 4.6
End of month 5
24 weeks
Month 6
About 5.5
End of month 6
28 weeks
Month 7
About 6.4
End of month 7
32 weeks
Month 8
About 7.4
End of month 8
36 weeks
Month 9
About 8.3
End of month 9
Popular week counts converted to months.
12 weeks: about 3 months. You are near the end of the first trimester. See 12 weeks pregnant.
20 weeks: about 4.5 to 5 months, the halfway point of a 40-week pregnancy. See 20 weeks pregnant.
24 weeks: about 5.5 months, so you are in your 6th month. See 24 weeks pregnant.
36 weeks: about 8 to 8.5 months, so you are in your 9th month with about 4 weeks to your due date. See 36 weeks pregnant.
How to Convert Pregnancy Weeks to Months Yourself
If you have a week count and no chart, there are two quick ways to answer “how many months pregnant am I?”
Months completed: multiply your weeks by 7 to get days, then divide by 30.44. Example: 18 weeks x 7 = 126 days; 126 / 30.44 = about 4.1 months completed. You are in your 5th month.
Rough rule: divide your weeks by 4.3. 30 weeks / 4.3 = about 7 months completed, so you are in your 7th or 8th month depending on the chart.
Why not just divide by 4? Because only February has exactly 4 weeks (in a non-leap year). Every other month has 30 or 31 days, so dividing by 4 makes you sound further along than you are. Forty weeks divided by 4 is 10 “months,” while 280 days divided by 30.44 is about 9.2 calendar months. That is the whole reason the nine-versus-ten-months debate exists.
If you are not sure how many weeks you are, your provider can confirm it. In the very early weeks, a pregnancy test and hCG levels come up a lot, but your provider dates the pregnancy from your LMP, and an early ultrasound can refine that date (more on that below).
Trimesters by Week: Why Sources Disagree on Week 13
Every source agrees there are three trimesters. They do not all draw the lines at the same week:
Source
First trimester
Second trimester
Third trimester
Office on Women’s Health (HHS)
Week 1 to week 12
Week 13 to week 28
Week 29 to week 40
Cleveland Clinic, Pregnancy
Until 13 weeks
Weeks 13 to 28
Week 29 to 40
Cleveland Clinic, Fetal Development
Conception to 12 weeks (it also calls the first 13 weeks the first trimester)
Weeks 13 to 28 (by its month headings)
Weeks 29 onward (by its month headings)
Trimester boundaries as stated by each source.
So week 13 sits right on the border. The Office on Women’s Health puts it at the start of the second trimester, while Cleveland Clinic describes the first trimester as lasting until 13 weeks. Cleveland Clinic notes each trimester is about three months, or about 13 weeks. In practice, the label matters much less than your week count.
Both answers show up, and both are about the same 40 weeks. Cleveland Clinic puts it plainly: depending on how you count, you could be pregnant for either nine months or 10 months.
Nine months: if you use calendar months. Cleveland Clinic describes pregnancy as 40 weeks or 280 days, and StatPearls says an average pregnancy lasts 280 days from the first day of the LMP. Divided by the average calendar month, that is about 9.2 months.
Ten months: if every month is exactly 4 weeks. Cleveland Clinic’s fetal development guide uses ten 4-week months for this reason.
Remember that the count starts about two weeks before conception (see the next sections), so the baby has been growing for a little less time than the week count suggests.
How Your Due Date Is Calculated
Your estimated due date (EDD) is the anchor for every week and month count. Here is how it is set, according to the sources we read:
Start from your LMP: Cleveland Clinic says providers usually date a pregnancy from the date of your last menstrual period. MedlinePlus defines gestational age as measured in weeks from the first day of the last menstrual cycle to the current date.
Add 280 days: StatPearls (an NIH clinical reference) says an average pregnancy lasts 280 days from the first day of the LMP.
Naegele’s rule: the classic shortcut. StatPearls describes it as subtracting 3 months and adding 7 days; Cleveland Clinic phrases it as adding seven days to your LMP, counting back three months and adding a year.
Its built-in assumption: StatPearls notes that Naegele’s rule assumes a cycle of exactly 28 days with ovulation on day 14. If your cycles are longer, shorter or irregular, the LMP date alone can be off.
Early ultrasound. StatPearls says measuring the embryo’s crown-rump length is the best dating method from 8 6/7 to 13 6/7 weeks, with a margin of error of plus or minus 5 to 7 days, and that later scans use other fetal measurements instead. It reports that if the LMP date and the first-trimester ultrasound date fall within 7 days of each other, the LMP date is kept. StatPearls adds that the American College of Obstetricians and Gynecologists (ACOG) considers dating ultrasounds done after 22 weeks suboptimal for measuring gestational age, which is one reason an early scan carries the most weight.
A due date is an estimate. Cleveland Clinic says the chances of giving birth on your estimated due date are only about 5%. That is why the term categories below cover a range of weeks rather than a single day.
If your LMP started on
Add 7 days
Count back 3 months
Estimated due date
January 10
January 17
October 17 of the previous year
October 17 of the same year
March 1
March 8
December 8 of the previous year
December 8 of the same year
June 20
June 27
March 27 of the same year
March 27 of the next year
Worked examples of the Naegele’s rule shortcut as Cleveland Clinic describes it (add 7 days, count back 3 months, add a year). Your provider’s date may differ, especially after an early ultrasound.
Gestational Age vs Fetal Age: The Two-Week Gap
This is the part most charts skip, and it explains why “4 weeks pregnant” can feel too early to be pregnant at all.
Gestational age: the number your provider uses. Cleveland Clinic says gestational age describes the pregnancy, not the fetus, and is written as weeks and days, such as 22 weeks and 3 days, or 22 3/7.
Fetal (conception) age: how long since conception. Cleveland Clinic says gestational age is about two weeks ahead of when conception actually occurs, and that fetal age is typically not a measurement your provider will use.
Cleveland Clinic also explains the practical effect: ovulation does not happen until about two weeks into your cycle, so if you take a pregnancy test two weeks after ovulation, you are already about “four weeks pregnant.” In other words, the first two weeks of the count happen before you are actually pregnant.
Every week and month on this page uses gestational age, which is also what our week-by-week guides use.
Early Term, Full Term, Late Term and Post-Term
Near the end, your provider will use term labels rather than months. In 2013, ACOG and the Society for Maternal-Fetal Medicine recommended these categories, as reported by the NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). Cleveland Clinic lists the same ranges today.
Category
Gestational age
Rough month (this chart)
Preterm
Before 37 weeks (MedlinePlus)
Month 9 or earlier
Early term
37 weeks 0 days through 38 weeks 6 days
Month 9
Full term
39 weeks 0 days through 40 weeks 6 days
End of month 9
Late term
41 weeks 0 days through 41 weeks 6 days
Past the due date
Post-term
42 weeks 0 days and beyond
Past the due date
Term definitions. Sources: NICHD (ACOG and SMFM categories), Cleveland Clinic, MedlinePlus.
MedlinePlus adds a wider view: a normal pregnancy can range from 38 to 42 weeks, infants born before 37 weeks are considered premature, and infants born after 42 weeks are considered postmature. That range is one more reason a due date is best treated as a window.
For the final stretch, our 36 weeks pregnant guide covers what to expect as you approach early term.
Why Providers Count in Weeks, Not Months
Weeks are exact; months are not. A month can be 28, 29, 30 or 31 days long, so “5 months pregnant” could mean anything from about 18 to 22 weeks. A week count, written as weeks plus days (such as 22 3/7), pins the pregnancy to a single day, and Cleveland Clinic notes this weeks-and-days format is how gestational age is recorded.
A practical tip that most charts miss: write your official due date and your gestational age at your first ultrasound in your phone notes. If you ever see a new provider, go to urgent care or get asked “how far along are you?”, give the answer in weeks and days rather than months. It avoids the exact mix-ups this chart is trying to untangle.
Call your provider any time something feels wrong, at any week or month. The Centers for Disease Control and Prevention (CDC) Hear Her campaign lists urgent maternal warning signs, for during pregnancy and the year after delivery, and says to seek medical care immediately if you have any of them:
A headache that won’t go away or gets worse over time.
Dizziness or fainting.
Changes in your vision.
A fever of 100.4 F or higher.
Extreme swelling of your hands or face.
Trouble breathing, chest pain or a fast-beating heart.
Severe nausea and throwing up.
Severe belly pain that doesn’t go away.
Your baby’s movement stopping or slowing during pregnancy.
Vaginal bleeding or fluid leaking during pregnancy.
Severe swelling, redness or pain of your leg or arm.
Overwhelming tiredness.
Thoughts about harming yourself or your baby.
In early pregnancy, know the ectopic pregnancy warning signs. Mayo Clinic says the first warning signs are often light vaginal bleeding and pelvic pain, and that you should seek emergency medical help if you have severe abdominal or pelvic pain accompanied by vaginal bleeding, extreme lightheadedness or fainting, or shoulder pain.
For an emergency, call 911 or go to the nearest emergency room. If you are not sure whether a symptom is serious, call your provider and describe it; do not wait to see if it passes. This article is general education, not medical advice.
Frequently Asked Questions
How do you convert pregnancy weeks to months?
Multiply your weeks by 7 to get days, then divide by about 30.4, the average calendar month. That gives months completed. For example, 24 weeks is about 5.5 months completed, so you are in your 6th month. Charts group weeks slightly differently, so treat any month label as approximate.
How many months is 24 weeks pregnant?
At 24 weeks, about 5.5 months have passed, so you are in your 6th month of pregnancy. On a chart that uses 4-week months, such as Cleveland Clinic’s fetal development guide, 24 weeks is the end of month 6. You are in the second trimester.
Is 30 weeks 7 or 8 months pregnant?
It depends on the chart. At 30 weeks, about 6.9 calendar months have passed, so many charts place you in month 7. Charts that use 4-week months, like Cleveland Clinic’s, put week 30 in month 8. Either way, you are in the third trimester.
Is pregnancy actually 9 months or 10 months?
Both are used. Pregnancy lasts about 40 weeks, or 280 days, from the first day of your last period. In calendar months that is a little over 9 months. If every month is counted as exactly 4 weeks, it comes to 10. Cleveland Clinic says you could be pregnant for either.
How many months pregnant am I at 20 weeks?
At 20 weeks you are about 4.6 months along, in your 5th month and at the halfway point of a 40-week pregnancy. On charts with 4-week months, 20 weeks is the end of month 5. The Office on Women’s Health places week 20 in the second trimester.
Why is pregnancy counted from the last period and not conception?
The first day of your last period is a date most people know, while the exact day of conception usually is not. Cleveland Clinic explains that gestational age runs about two weeks ahead of conception, which is why you are already about 4 weeks pregnant when a test turns positive.
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