Enter the first day of your last menstrual period to see how many weeks pregnant you are, how many days are left and your estimated due date.
Pregnancy is dated from the first day of your last menstrual period, not from conception. The estimated due date is 280 days (40 weeks) after that day, or 266 days after conception when the conception date is known (UpToDate, 2025). In a regular 28-day cycle, gestational age is therefore 14 days more than the time since conception.
The calculator counts the days between your last period and today and divides by 7, showing the result as weeks + days: 24 weeks 3 days means you are 24 weeks pregnant. A due date is an estimate, not a deadline; only about 4% of people give birth on their due date.
With Naegele's rule you count back three months from the first day of your last period and add seven days. If your last period started on February 20, the estimated due date is November 27 (UpToDate, 2025). The rule assumes a 28-day cycle with conception on day 14.
Because months have different lengths, working the rule out by hand can give a date 1–3 days different from last period + 280 days on some dates; our calculator uses the standard definition, last period + 280 days, and handles leap years through the calendar. If the result is 42 weeks 0 days or more, the tool shows a postterm note; for a future date or a result over 45 weeks, it asks you to check the date. Trimesters are shown as follows:
| Trimester | Gestational age |
|---|---|
| First | 0 to 13 weeks 6 days |
| Second | 14 weeks 0 days to 27 weeks 6 days |
| Third | 28 weeks 0 days to birth |
The calculation assumes that the first day of your last period is known exactly and that your cycle lasts 28 days. The result can differ from your true gestational age when (UpToDate, 2025):
In these situations ultrasound takes priority. Crown-rump length (CRL) measured in the first trimester is the most accurate method: ±5 days up to 8 weeks 6 days and ±7 days between 9 weeks 0 days and 13 weeks 6 days. The due date confirmed or set by the earliest ultrasound done at or before 21 weeks 6 days becomes your due date; later scans do not change it. Without an ultrasound before 22 weeks 0 days that confirms the dates, a pregnancy is considered “suboptimally dated” (ACOG Committee Opinion 700, 2017). The Turkish Ministry of Health guideline also asks that gestational age be set from ultrasound reports when the last period is unknown or uncertain (Turkish Ministry of Health, 2026). If you have an ultrasound-based week, use the last period finder.
If in the first trimester the baby measures clearly smaller than your period dates suggest, this is not always explained by late ovulation; your doctor may reassess the pregnancy's development after a short interval (UpToDate, 2025).
When the date of fertilization is known for certain, the due date is calculated from it (UpToDate, 2025):
| IVF cycle | Estimated due date |
|---|---|
| Fresh cycle | Egg retrieval / fertilization date + 266 days |
| Frozen day-3 embryo transfer | Transfer date + 263 days |
| Frozen day-5 embryo (blastocyst) transfer | Transfer date + 261 days |
This tool asks for a last menstrual period. After IVF you can enter your transfer date and the gestational age on that day (see the questions below) into the last period finder to get the same calendar.
Checkups and tests are scheduled in weeks; months are only a rough guide. The table below is simple arithmetic that counts each month as 30.44 days. For details of your week, see the week-by-week pregnancy pages.
| Week of pregnancy | Month of pregnancy | Trimester |
|---|---|---|
| Week 4 | Month 1 into month 2 | First |
| Week 8 | Month 2 into month 3 | First |
| Week 12 | Month 3 | First |
| Week 16 | Month 4 | Second |
| Week 20 | Month 5 | Second |
| Week 24 | Month 6 | Second |
| Week 28 | Month 7 | Third |
| Week 32 | Month 8 | Third |
| Week 36 | Month 9 | Third |
| Week 40 | Month 10 | Third |
A baby is considered “term” from 37 weeks, and the term period is divided as follows (UpToDate, 2025):
To see these weeks on a calendar, use the advanced pregnancy calendar. The calculator does not make a diagnosis; your doctor confirms your gestational age and due date together with ultrasound findings.
Count the days from the first day of your last period to today and divide by 7: the whole number is the weeks and the remainder is the days. For example, 171 days is 24 weeks 3 days. If your dates were corrected by ultrasound, use the ultrasound-based week.
From the last period. Gestational age starts on the first day of the last menstrual period; “embryonic age”, counted from conception, is 14 days less in a 28-day cycle and is not used in pregnancy care (UpToDate, 2025).
You may not have ovulated on day 14, your cycle may be longer, or the date of your last period may be uncertain. If the difference on a first-trimester scan is larger than expected, the due date is set by ultrasound, and later scans do not change it.
Because the fertilization date is known, the calculation starts from it: the due date is 266 days after egg retrieval in a fresh cycle, 263 days after a frozen day-3 embryo transfer and 261 days after a frozen day-5 embryo (blastocyst) transfer (UpToDate, 2025). On the transfer day, gestational age therefore counts as 2 weeks 3 days for a day-3 embryo and 2 weeks 5 days for a day-5 embryo; enter the transfer date with that week and day into the last period finder to see your current week and due date.
Low: about 4% of people give birth on their estimated due date, partly because of the limits of dating methods and partly because of natural variation in when labor starts (UpToDate, 2025).
Week 12 is about 2 months and 23 days after the last period, which falls in the third month of pregnancy and within the first trimester.
Last updated: .