The short answer: pregnancy is counted as 40 weeks from the first day of your last period, and there are two common ways to turn those weeks into months. The count this site and the Baby Novum app use — and most English-language charts — groups the weeks into nine months of four or five weeks: month 1 is weeks 1–4, month 5 is weeks 18–22, month 6 is weeks 23–27, month 7 is weeks 28–31 and month 9 is weeks 36–40. The other count, used in Germany and in Ukrainian and Russian practice, splits the 280 days into ten four-week months. Trimesters run weeks 1–13, 14–27 and 28–40. One detail changes the answer by a whole month: "week 27" is the 27th week, written 26+0 to 26+6, while "27 weeks pregnant" usually means 27 weeks completed — which is already week 28.
How many months is 27 weeks pregnant?
It depends on what "27 weeks" means, and that single detail is why search results disagree. Week 27 — the 27th week, written 26+0 to 26+6 in your notes — is month 6 of 9 and the last week of the second trimester. 27 weeks completed — 27+0 to 27+6, which is how most people mean "27 weeks pregnant" — is already week 28: month 7, and the first week of the third trimester.
| You are in | In your notes | Month (of 9) | 4-week month (of 10) | Trimester | Calendar months since your period |
|---|---|---|---|---|---|
| Week 27 | 26+0 – 26+6 | 6 | 7 | 2 (last week) | about 6 |
| Week 28 | 27+0 – 27+6 | 7 | 7 | 3 (first week) | about 6¼ |
The same one-week shift applies to every row of the chart, which is why it carries an "In your notes" column. Scan reports, maternity notes and the German Mutterpass all write completed weeks and days; the week pages on this site, the chart and the Baby Novum app all name the week you are in.
Nine months or ten? The two ways to count
Nine months of four or five weeks. Forty weeks do not divide evenly into nine, so the weeks are grouped as 1–4, 5–8, 9–13, 14–17, 18–22, 23–27, 28–31, 32–35 and 36–40. This is the count behind the Month column on every week page here and in the Baby Novum app, and the one most English-language pregnancy charts use.
Ten months of four weeks. The 280 days from the first day of the last period split exactly into ten 28-day months. Germany's federal family-planning site, familienplanung.de, puts it as „40 Wochen oder zehn Mondmonate (Lunarmonate) mit jeweils 28, also insgesamt 280 Tagen“ — 40 weeks, or ten lunar months of 28 days — and the obstetric month of Ukrainian and Russian practice is the same four weeks. In this count week 27 and week 28 both fall in month 7.
Calendar months are the third answer and the least tidy: 280 days is about 9.2 calendar months, and the exact figure depends on which months your pregnancy spans. The converter above counts real calendar months from your own last period.
| Month | Nine months of 4–5 weeks | Ten months of 4 weeks |
|---|---|---|
| 1 | Weeks 1–4 | Weeks 1–4 |
| 2 | Weeks 5–8 | Weeks 5–8 |
| 3 | Weeks 9–13 | Weeks 9–12 |
| 4 | Weeks 14–17 | Weeks 13–16 |
| 5 | Weeks 18–22 | Weeks 17–20 |
| 6 | Weeks 23–27 | Weeks 21–24 |
| 7 | Weeks 28–31 | Weeks 25–28 |
| 8 | Weeks 32–35 | Weeks 29–32 |
| 9 | Weeks 36–40 | Weeks 33–36 |
| 10 | — | Weeks 37–40 |
When does each trimester start?
This site and the Baby Novum app use weeks 1–13, 14–27 and 28–40, the same split as the NHS week-by-week guide. Forty weeks do not divide into three equal blocks, so published sources place the boundaries a week apart:
| Source | First trimester | Second trimester | Third trimester |
|---|---|---|---|
| Baby Novum (this site and the app) | Weeks 1–13 | Weeks 14–27 | Weeks 28–40 |
| NHS week-by-week guide | Weeks 1–13 | Weeks 14–27 | From week 28 |
| Cleveland Clinic | — | — | Weeks 28–40 |
| NICHD, US National Institutes of Health | Weeks 1–12 | Weeks 13–28 | Weeks 29–40 |
The label matters less than what changes around it. From the end of the second trimester appointments come closer together, blood pressure matters more, and from 28 weeks kick counts become a daily habit. The second trimester guide and the third trimester guide go week by week.
Reading 27+3 in your notes
Scan reports and maternity notes write the stage of pregnancy as completed weeks plus days. 27+3 means 27 whole weeks and 3 days since the first day of your last period. Because week 1 covers 0+0 to 0+6, 27+3 falls in week 28. A reading of 26+3 is week 27. It is the most common reason two sources give you different months for what looks like the same number.
Why pregnancy is counted in weeks
Dating runs from the first day of the last menstrual period, and the due date is 280 days — 40 weeks — after it. Conception usually happens about two weeks later, so on the day you conceive you are already counted as two weeks pregnant. Weeks are used rather than months because months are uneven and too coarse: a great deal changes from one week to the next, and every routine scan and test is timed to a window only a few weeks wide. If a dating scan moves your due date, every week and month here moves with it.
Contact your midwife or maternity unit the same day, whatever week or month you are in, if you have:
- Reduced or changed baby movements after 28 weeks
- Vaginal bleeding at any stage
- A severe headache, visual disturbance, or sudden swelling of the face and hands — the signs of pre-eclampsia
- Fluid leaking from the vagina, or regular painful tightenings, before 37 weeks
- Severe abdominal pain, a fever, or pain on passing urine
None of these waits for the next scheduled appointment.
Want every week dated? The pregnancy calendar turns your due date into forty dated weeks with every scan window, and the due date calculator works the date out from your last period, a conception date or an IVF transfer.
Related tools and guides
FAQ — Pregnancy weeks to months
How many months is 27 weeks pregnant?
Week 27 — written 26+0 to 26+6 in your notes — is month 6 of 9 and the last week of the second trimester. If your notes say 27+ something, you have completed 27 weeks and are in week 28: month 7 and the first week of the third trimester. Counted in four-week months, both fall in month 7 of 10, and about six calendar months have passed since your last period.
How many months is 20 weeks pregnant?
Week 20 — 19+0 to 19+6 in your notes — is month 5 of 9 (month 5 runs from week 18 to week 22) and the fifth of ten four-week months: the halfway point of a 40-week pregnancy. If your notes say 20+ something, you are in week 21 — still month 5 of 9, but the sixth four-week month. About four and a half calendar months have passed since the first day of your last period.
How many weeks is 6 months pregnant?
In the nine-month count used on this site and in the Baby Novum app, month 6 is weeks 23 to 27. In the count of ten four-week months it is weeks 21 to 24. Six calendar months after the first day of your last period you are about 26 weeks pregnant.
Is pregnancy 9 months or 10 months?
Both, depending on the month you count in. Forty weeks from the first day of your last period is 280 days — just over nine calendar months, or exactly ten four-week "lunar" months. English-language charts usually group the weeks into nine months of four or five weeks, while in Germany, Ukraine and Russia the four-week month is the usual unit.
When does the third trimester start?
At week 28 on this site, in the Baby Novum app and in the NHS week-by-week guide, which makes week 27 the last week of the second trimester. Sources differ by about a week: the US National Institutes of Health, for example, counts it from week 29.
What does 27+3 mean in my notes?
It means 27 completed weeks and 3 days since the first day of your last period — the notation scan reports and maternity notes use. You are in week 28, because week 1 covers 0+0 to 0+6. The "In your notes" column of the chart lines the two up for every week.
How many months is 40 weeks?
Forty weeks is 280 days: about 9.2 calendar months, month 9 in the nine-month count, and the tenth four-week month. Your due date is simply the end of week 40 — anything from 37 to 42 weeks is a normal length of pregnancy.
Why do doctors count pregnancy in weeks rather than months?
Because a month is too coarse and months are not equal in length. A great deal changes in a single week, and every scan and test is timed to a window only a few weeks wide. Weeks are counted from the first day of your last period, about two weeks before conception — which is why you are counted as two weeks pregnant on the day you conceive.
Sources
- ACOG Committee Opinion 700 — Methods for Estimating the Due Date
- NHS — 28 weeks pregnant: the third trimester begins
- Cleveland Clinic — Third trimester of pregnancy
- NICHD (NIH) — Pregnancy: the three trimesters
- familienplanung.de — Schwangerschaftswochen zählen (10 Mondmonate × 28 Tage)
Pregnancy weeks to months — at a glance
Pregnancy is dated as 40 weeks, or 280 days, from the first day of the last menstrual period, and there are two standard ways to express that in months. The nine-month count used on this site, in the Baby Novum app and in most English-language charts groups the weeks as 1–4, 5–8, 9–13, 14–17, 18–22, 23–27, 28–31, 32–35 and 36–40; the ten-month count, usual in Germany and in Ukrainian and Russian practice, splits the 280 days into ten four-week months. Trimesters run weeks 1–13, 14–27 and 28–40 here and in the NHS guide, with other sources placing the boundaries a week earlier or later. The biggest source of confusion is the week itself: "week 27" is the 27th week, written 26+0 to 26+6, while 27 completed weeks — 27+0 onwards — is already week 28. That is the difference between month 6 and month 7, and between the second trimester and the third. Use the chart's "In your notes" column, or the converter, to line your own figure up.
Baby Novum is an informational and tracking tool, not a medical device, and does not provide medical diagnosis or treatment. Always consult your doctor or midwife about your pregnancy.