Pregnancy Calculator
Find your due date, how many weeks pregnant you are and your trimester dates from your last period, conception date, IVF transfer date, ultrasound or a known due date. Example: a first day of last period of January 1, 2026 gives a due date of October 8, 2026.
Only used for the ultrasound option
Show steps
How to use the Pregnancy Calculator
- Choose what you know: the first day of your last period, your conception date or your due date.
- Enter the date and your average cycle length (28 days if unsure).
- Press Calculate to see your due date, weeks pregnant, trimester and key dates.
Formula and method
Pregnancy is dated from the first day of the last menstrual period (LMP) and lasts about 40 weeks (280 days). Due date = LMP + 280 days (Naegele’s rule, adjusted by your cycle length minus 28 days). Conception usually happens about 14 days after the LMP in a 28-day cycle, so due date from conception = conception date + 266 days. Gestational age is the number of days since the LMP, shown as weeks and days.
Pregnancy trimesters and weeks
| Stage | Weeks | Days from LMP |
|---|---|---|
| First trimester | 0 to 13 weeks 6 days | 0 to 90 |
| Second trimester | 14 to 27 weeks 6 days | 91 to 188 |
| Third trimester | 28 weeks to birth | 189 to 294 |
| Early term | 37 to 38 weeks 6 days | 259 to 272 |
| Full term | 39 to 40 weeks 6 days | 273 to 286 |
| Late term | 41 weeks to 41 weeks 6 days | 287 to 293 |
Example: LMP on January 1, 2026
Adding 280 days to January 1, 2026 gives October 8, 2026. Conception is estimated around January 15, the second trimester begins on April 9 (14 weeks) and the third on July 16 (28 weeks). Use the due date calculator for a simple date-only estimate and the ovulation calculator to plan around your fertile window.
How accurate is a due date?
Only about 4 in 100 babies are born on the estimated due date, and most arrive within two weeks either side. Irregular cycles, a late ovulation or IVF dates can change the estimate. A first-trimester ultrasound measuring the baby’s length is the most accurate way to date a pregnancy, and your clinician will use it to confirm or correct this estimate.
What month of pregnancy am I in?
| Month | Weeks |
|---|---|
| Month 1 | Weeks 1 to 4 |
| Month 2 | Weeks 5 to 8 |
| Month 3 | Weeks 9 to 13 |
| Month 4 | Weeks 14 to 17 |
| Month 5 | Weeks 18 to 21 |
| Month 6 | Weeks 22 to 26 |
| Month 7 | Weeks 27 to 30 |
| Month 8 | Weeks 31 to 35 |
| Month 9 | Weeks 36 to 40 |
A pregnancy lasts about 40 weeks, which is nine calendar months plus about one week, because months are longer than four weeks.
IVF, ultrasound and conception dates
With IVF the date of transfer is known exactly. A day-5 blastocyst transfer is dated as 261 days before the due date, and a day-3 embryo transfer as 263 days. For natural conception, the due date is conception plus 266 days. When you choose the ultrasound option, enter the scan date and the weeks and days your report gives; the calculator counts forward to 40 weeks from that.
When can the estimated date change?
| Gestational age at first ultrasound | Difference from period-based date that usually leads to re-dating |
|---|---|
| Up to 8 weeks 6 days | More than 5 days |
| 9 weeks to 13 weeks 6 days | More than 7 days |
| 14 weeks to 15 weeks 6 days | More than 7 days |
| 16 weeks to 21 weeks 6 days | More than 10 days |
| 22 weeks to 27 weeks 6 days | More than 14 days |
| 28 weeks and beyond | More than 21 days |
These are the thresholds published by the American College of Obstetricians and Gynecologists (Committee Opinion 700). Your clinician decides whether to change a due date; this table is for background only.
Frequently asked questions
How do I calculate my due date?
Add 280 days (40 weeks) to the first day of your last period, or add 266 days to the conception date.
How many weeks pregnant am I?
Count the days since the first day of your last period and divide by 7. The calculator shows the result in weeks and days.
When does the second trimester start?
At 14 weeks, about 98 days after the first day of your last period.
How many months is 40 weeks?
About nine months plus a week. Pregnancy is counted in weeks because calendar months vary in length.
Do I count from the first day of my period or from conception?
Pregnancy is dated from the first day of the last period, so you are about two weeks pregnant at conception. That is why gestational age runs ahead of the baby’s actual age.
Why is my ultrasound due date different?
Ovulation does not always happen on day 14. Early ultrasound measures the baby’s size, which is a more reliable guide than the period date, and your clinician may move the due date.
When can I take a pregnancy test?
Most home tests are most reliable from the first day of a missed period. Testing earlier can give a false negative because hCG is still low.
What chance is there of delivering on the due date?
Only about 4 in 100 babies are born on the estimated date. Most arrive in the three weeks around it.
Is this a medical diagnosis?
No. It is an estimate. Confirm dates and care with your doctor or midwife.
Sources
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