How are weeks of pregnancy counted?
Gestational age is counted from the first day of your last menstrual period (LMP), which is treated as day 0, and not from the day of conception. Conception normally happens shortly after ovulation, which in a 28-day cycle is around two weeks after the LMP. So when you are "4 weeks pregnant" the embryo is only about two weeks old, and a missed period lands at roughly 4 weeks 0 days. The convention exists because very few people know their ovulation date, while most can remember or look up when their last period began. Ages are written as weeks plus days, for example "13 weeks 2 days", and a week is always 7 days.
Naegele's rule and the formula behind this tool
The method is named after the German obstetrician Franz Naegele. It sets the estimated due date (EDD) at LMP + 280 days (40 weeks). On a calendar it is often remembered as "subtract three months, add seven days". The 280 days assume a 28-day cycle with ovulation on day 14, which makes the baby about 266 days (38 weeks) old at the due date counted from conception.
When your cycle is not 28 days, ovulation moves, so the rule is adjusted by (cycle length β 28) days. This calculator does exactly that: the due date is the LMP plus 280 plus the adjustment, and today's gestational age is (days since LMP β adjustment) divided by 7, shown as whole weeks and leftover days. The adjustment rests on the assumption that ovulation happens 14 days before the next period, so it is unreliable if your cycles vary a lot from month to month. The input accepts cycles of 20 to 45 days; anything outside that range is treated as 28.
| Average cycle | Estimated ovulation (day of cycle) | Due date adjustment | Due date (LMP + days) |
|---|---|---|---|
| 21 days | Day 7 | -7 days | 273 days |
| 24 days | Day 10 | -4 days | 276 days |
| 26 days | Day 12 | -2 days | 278 days |
| 28 days | Day 14 | 0 days | 280 days |
| 30 days | Day 16 | +2 days | 282 days |
| 32 days | Day 18 | +4 days | 284 days |
| 35 days | Day 21 | +7 days | 287 days |
| 38 days | Day 24 | +10 days | 290 days |
| 40 days | Day 26 | +12 days | 292 days |
Worked examples
Example 1: 28-day cycle, LMP on 10 January 2026. Adding 280 days gives a due date of 17 October 2026. On 20 March 2026, 69 days have passed, and 69 Γ· 7 is 9 weeks with 6 days left over, so you would be 9 weeks 6 days.
Example 2: 35-day cycle, same LMP. The adjustment is 35 β 28 = +7 days, so the due date is 287 days after the LMP, which is 24 October 2026. On the same 20 March the count becomes 69 β 7 = 62 days, or 8 weeks 6 days. A longer cycle means ovulation probably happened later, so the same calendar day shows almost a week less.
Example 3: 30-day cycle, LMP on 4 May 2026. The adjustment is +2 days, so the due date is LMP + 282 days, which is 10 February 2027. Estimated ovulation is day 16, on 20 May 2026, and 37 weeks (plus the 2-day adjustment) falls on 20 January 2027.
Why an ultrasound can change the due date
If you are unsure of your LMP or your cycles are irregular, the calculated age can drift from reality. An early ultrasound measures the embryo's crown-rump length (CRL), and in the first trimester babies are so similar in size that this narrows the dating error to a few days. That is why early scan dating is generally trusted over LMP dating when the two disagree. As pregnancy advances, babies differ more in size and the margin of error on a scan grows.
The American College of Obstetricians and Gynecologists (ACOG, Committee Opinion 700, 2017) suggests redating by ultrasound when the gap between the LMP-based and scan-based due dates exceeds the threshold below. Your own clinic may apply different rules, so treat the table as an explanation of the idea.
| Gestational age at the scan (by LMP) | Redate if the difference exceeds |
|---|---|
| Up to 8 weeks 6 days | 5 days |
| 9 weeks 0 days to 13 weeks 6 days | 7 days |
| 14 weeks 0 days to 15 weeks 6 days | 7 days |
| 16 weeks 0 days to 21 weeks 6 days | 10 days |
| 22 weeks 0 days to 27 weeks 6 days | 14 days |
| 28 weeks 0 days or later | 21 days |
Example where the scan wins: your LMP is 10 January 2026 (due 17 October 2026), but a scan on 20 March 2026 measures 8 weeks 0 days. The scan-based due date is 20 March plus (280 β 56) days, which is 30 October 2026. That is 13 days after the LMP-based date, and by LMP you would be 9 weeks 6 days at the scan, where the limit is 7 days. The difference is larger, so the due date would typically be moved to 30 October.
Example where the LMP stays: your LMP is 1 February 2026 (due 8 November 2026) and a scan on 10 April 2026 measures 10 weeks 3 days. The scan-based due date is 3 November 2026, only 5 days earlier. By LMP you were 9 weeks 5 days at the scan, the limit is 7 days, and 5 is within it, so the LMP date normally stays. You can enter the same figures in the box above to see this comparison done for you. The final decision always belongs to your clinician.
Trimesters and term
- First trimester: 0 weeks to 13 weeks 6 days. Nausea is common and the organs are forming, so clinicians are especially careful about medicines and infections in this period.
- Second trimester: 14 weeks 0 days to 27 weeks 6 days. Many people feel their best here, though experiences differ a great deal.
- Third trimester: 28 weeks 0 days until birth. Weight gain, swelling and warning signs of early labor or high blood pressure deserve attention.
ACOG divides delivery timing into early term (37 weeks 0 days to 38 weeks 6 days), full term (39 weeks 0 days to 40 weeks 6 days), late term (41 weeks 0 days to 41 weeks 6 days) and post-term (42 weeks 0 days and beyond). Anything before 37 weeks is preterm. The "Full term (37 wk) begins" chip above is the date you reach 37 weeks 0 days by LMP, adjusted for cycle length. Only a small share of babies arrive on the due date itself, so it is more realistic to treat it as the middle of a window of two to three weeks.
If you are not sure of your last period
The date to use is the first day of real bleeding, not the last day. If you kept no record, a calendar or cycle-tracking app, ovulation test strips and the date of your first positive test can give you a rough range. Pass that range on to your clinician as it is rather than picking one day, because they will probably rely on an early scan to settle the dating. Light spotting that is clearly lighter than your normal period may be implantation bleeding, so the usual reference is the last period that looked like your normal ones. If the date is shaky, trust the due date your clinician gives you over this tool's D-day counter.
Weight gain during pregnancy (for background)
The US Institute of Medicine (2009) gave total weight gain ranges by pre-pregnancy BMI for a single baby: about 12.5 to 18 kg for underweight (BMI below 18.5), 11.5 to 16 kg for normal weight (18.5 to 24.9), 7 to 11.5 kg for overweight (25 to 29.9) and 5 to 9 kg for obesity (30 and above). These use the WHO categories, which are the ones used in the BMI calculator. Individual targets are set with your care team, and the usual calorie-deficit planning for weight loss should not be applied during pregnancy.
Limits and common misunderstandings
- Uncertain LMP or bleeding early on: mistaking implantation bleeding for a period shifts the dates.
- Irregular cycles (for example after stopping hormonal contraception, while breastfeeding, or with conditions such as PCOS): ovulation can vary widely, so LMP-based weeks may be off.
- IVF pregnancies: dating is worked out from the embryo transfer date and embryo age, so this calculator does not apply.
- Not a pregnancy test: the tool does not tell you whether you are pregnant. Use a test and see a clinician.
- "280 days is the exact length of pregnancy": it is a convention close to the average, and individual pregnancies differ.
- Twins and multiples: they tend to be delivered earlier than 40 weeks, and the formula here is for a single baby.
- It cannot say anything about how the baby is growing or whether a scan finding is normal.
What to do with the result
- Bring the calculated weeks and due date to your first appointment so they can be checked against the scan.
- If your clinician changes the due date, use their date from then on.
- Do not apply a general calorie target to a pregnancy. The calorie calculator is meant for non-pregnant adults, so ask your care team about nutrition.
- To count down to the day, the D-day countdown calculator works with any target date.
Please note
This calculator is for general information and does not replace a medical consultation. Bleeding, severe abdominal pain, reduced baby movements or suspected leaking of fluid need prompt care regardless of any result shown here. Your dates are calculated in your browser and are not sent to a server.
Frequently Asked Questions
Why is the last period used instead of the ovulation date?
The real ovulation or conception date is hard to pin down, while the start of the last period is easy to identify. Dating from the LMP gives everyone the same reference point. As a result, conception usually happens about two weeks after day 0, but the count still begins at the LMP.
What if my cycles are irregular?
The accuracy of this calculator drops. An ultrasound in early pregnancy, ideally before about 13 weeks, is much more reliable for setting the due date, so your clinician's dating comes first.
My clinic gave a different number of weeks. Which is right?
Your clinic has probably adjusted the due date using ultrasound measurements, and in that case the clinic's date is the one to follow. The comparison box above can show how large the gap is.
Is 40 weeks nine months or ten?
Forty weeks is 280 days. Counting a month as four weeks gives ten months, while on the calendar it is roughly nine months and one week. That is why "nine months" and "ten months" are both heard.
Does a baby born before the due date have a problem?
From 37 weeks 0 days a baby is classed as term, which is within the normal range. Before 37 weeks it is preterm and needs medical attention. If you suspect early labor, contact your care provider right away.