How to use the pregnancy calculator
- Choose what to calculate from: the first day of your last period, a known conception date, an ultrasound scan, or an IVF embryo transfer.
- Enter that date. For a last period, add your average cycle length. For a scan, add the weeks and days written on the report. For IVF, choose a day-5 or day-3 embryo.
- Leave date to check on today, or pick a future date to see how far along you will be then.
- Read how many weeks and days pregnant you are, your due date, and the milestone table.
How pregnancy dates are calculated
Pregnancy is counted from the first day of the last menstrual period (LMP), about two weeks before conception. A full-length pregnancy is 280 days, or 40 weeks, from that date. Every method below works out that starting date, then counts forward:
The cycle adjustment is a common refinement of Naegele's rule (LMP + 280 days): it assumes ovulation 14 days before your next period. A 35-day cycle moves everything 7 days later. For IVF, this gives the dates the American College of Obstetricians and Gynecologists (ACOG) uses: a due date 261 days after a day-5 transfer, or 263 days after a day-3 transfer.
Worked example
The calculator opens with a last period 70 days before today and a 28-day cycle. If today is October 29, 2026, the last period started on August 20, 2026.
- Cycle adjustment: 28 − 28 = 0 days, so the start date stays August 20, 2026
- Days since then: 70, and 70 ÷ 7 = 10 weeks with nothing left over: 10 weeks, 0 days
- That is week 11 of pregnancy, in the first trimester
- Due date: August 20, 2026 + 280 days = Thursday, May 27, 2027
- Estimated conception: August 20 + 14 days = September 3, 2026
If the same period had come with a 35-day cycle, the start date would move 7 days later. You would be 9 weeks, 0 days, with a due date of June 3, 2027.
What the milestones mean
| Milestone | Weeks | What happens |
|---|---|---|
| End of the first trimester | 13w 6d | The first trimester runs to the end of week 13 |
| Anatomy scan | 18w to 22w 6d | Detailed mid-pregnancy ultrasound, usually offered in this window |
| Viability threshold | about 24w | Survival with intensive care becomes more likely |
| Early term | 37w to 38w 6d | Born from here on is no longer preterm |
| Full term | 39w to 40w 6d | The best time for birth when there are no complications |
| Late term / post-term | 41w / 42w | Your care team will usually discuss monitoring or induction |
These term definitions are ACOG's. Trimester boundaries vary slightly between sources; this calculator uses 0 to 13 weeks 6 days, 14 to 27 weeks 6 days, and 28 weeks onward.
Which method is most accurate?
An ultrasound in the first trimester, which measures the baby's crown-rump length, is the most accurate way to date a pregnancy. ACOG puts its accuracy at about 5 to 7 days. Dates from a last period are less certain, because ovulation does not always happen on the same cycle day. If your care team gives you a due date from a scan, use that date here with the ultrasound option. IVF dates are the most precise of all, because the embryo's age is known.
Frequently asked questions
Why am I "2 weeks pregnant" at conception?
Because pregnancy is counted from the first day of your last period, not from conception. Ovulation and conception usually happen about two weeks later, so by the time an egg is fertilized you are already counted as about 2 weeks pregnant.
What is the difference between "10 weeks" and "week 11"?
10 weeks, 0 days means ten full weeks have passed. You are then in your eleventh week, just as a baby who is 1 year old is in its second year. Apps and doctors use both styles, so check which one is meant.
Can my due date change?
Yes. If a first-trimester ultrasound gives a date more than about a week away from your last-period date, your doctor or midwife will usually change the due date to match the scan. Later scans are used to check growth, not to re-date the pregnancy.
How likely is my baby to arrive on the due date?
Not very. The due date is the midpoint of a range, and most babies are born somewhere between 37 and 42 weeks. Treat it as a guide for planning, and talk to your doctor or midwife about any questions on timing or symptoms.
Related calculators
- Due Date Calculator — Your estimated due date by Naegele's rule, adjusted for cycle length, or from a conception or IVF transfer date.
- Pregnancy Weight Gain Calculator — Recommended pregnancy weight gain for your pre-pregnancy BMI, one baby or twins, and the expected range for your week.
- Pregnancy Conception Calculator — Estimate your conception date and fertile window from your last period or due date, with the early pregnancy timeline.
- Ovulation Calculator — Your estimated ovulation date and fertile window for the next 6 cycles, from your last period and cycle length.
Results are estimates for general information and are not medical advice. Talk to a doctor or other qualified professional about your health. See our disclaimer.