Enter one date. Get your due date, the week you are in today, and a calendar of the scans and tests due each week.
Estimated due date
Your last period started on January 12. The clinic writes October 19 on your chart. A pregnancy app shows October 26. Neither is wrong. A due date is a calculation with one assumption inside it, and each method assumes something different. Choose the method to match what you know, and the week count, trimester, and key dates above follow from it.
The calculator runs one formula per method. The table shows what each one adds up and what it takes on faith.
| Method | Formula | Takes on faith | Best when |
|---|---|---|---|
| Last period | Period start + 280 days, shifted by cycle length minus 28 | Ovulation came 14 days before the next period | Cycles run regular and you know the first day |
| Conception date | Conception + 266 days | The date you entered is the real one | You tracked ovulation or had an insemination |
| Ultrasound | Scan date + (280 minus the age on the scan) | The scan measured the embryo correctly | A first trimester scan disagrees with your period date |
| IVF transfer | Transfer + 261 days (day 5), 263 (day 3), or 260 (day 6) | The embryo age is exact | Any fresh or frozen transfer |
Take a 28 day cycle. Add 280 days to the first day of bleeding and you land on the due date. A longer or shorter cycle moves ovulation, so the date moves by the same number of days.
The last line is the one to study. A conception date two weeks after the period lands on the same due date, because the 40 weeks were counted from the period all along. On the day of conception you are already at 2 weeks. At a missed period you are at 4 weeks, with an embryo two weeks old.
Care teams write age as weeks and days, so 27w6d is the last day of the second trimester. These are the boundaries used in obstetric records.
| Stage | Starts | Ends |
|---|---|---|
| First trimester | 0w0d | 13w6d |
| Second trimester | 14w0d | 27w6d |
| Third trimester | 28w0d | Birth |
| Early term | 37w0d | 38w6d |
| Full term | 39w0d | 40w6d |
| Late term | 41w0d | 41w6d |
| Post-term | 42w0d | Birth |
A birth before 37w0d counts as preterm. Elective delivery before 39w0d is discouraged, since early term babies face more breathing and feeding trouble than babies born at 39 weeks.
A first trimester ultrasound measures the embryo from crown to rump. Growth at this stage is so uniform, the measurement dates a pregnancy to within about five to seven days. A remembered period date rarely matches it. Care teams keep the period date unless the scan disagrees by more than the gap below.
| Age on the scan | Move the due date when scan and period differ by more than |
|---|---|
| Up to 8w6d | 5 days |
| 9w0d to 13w6d | 7 days |
| 14w0d to 15w6d | 7 days |
| 16w0d to 21w6d | 10 days |
| 22w0d to 27w6d | 14 days |
| 28w0d and later | 21 days |
Later scans measure size, and size varies from baby to baby, so they date poorly. If your team moved your date, pick Ultrasound above and enter the scan date with the age on the report.
About 4 in 100 babies are born on the estimated due date. Most arrive between 37 and 42 weeks. Read the date as the middle of a five week window, and plan leave, travel, and a hospital bag around week 37.
Where this calculator falls short
Download calendar adds one all-day event for every upcoming date in the list, plus your due date. Open the file in Google Calendar, Apple Calendar, or Outlook. Dates move when a scan redates the pregnancy, so download a fresh file after any change. Copy link stores your method and date in the URL, so a partner opens the same page.
Everything runs in your browser. The dates you type are never sent to a server and are gone when you close the tab.
Accuracy, methods, cycle length, and week counts.
About 4 in 100 babies arrive on the date. Most arrive between 37 and 42 weeks, so read the date as the middle of a window. A first trimester scan gives the tightest estimate, within about a week.
Use your last period when cycles run regular and you know the first day. Use IVF transfer for any transfer, fresh or frozen, since the embryo age is exact. Switch to Ultrasound when a first trimester scan differs from your period date by more than a week.
Pregnancy weeks count from the first day of your last period, about two weeks before conception. At a missed period you are at 4 weeks with a 2 week old embryo.
Yes. The formula assumes ovulation on day 14 of a 28 day cycle. A 35 day cycle pushes ovulation seven days later, so the due date moves seven days later. A 24 day cycle moves it four days earlier.
The first day of full bleeding. Spotting before it does not count. If the start is unclear, pick the first day you needed a pad or tampon.
Twelve completed weeks and three extra days, so you are in your 13th week. Clinics use this form on charts and scan reports.
The start date your dating implies is still ahead of today. For a period date, the date you entered is in the future. Check the year first, then the method.
Yes. Ovulation follows the LH surge by about a day, so enter the day after your first positive test and pick Conception date. Expect a margin of a couple of days either way.
No. Every calculation runs in your browser. Nothing goes to a server or stays after you close the tab. The link button writes your dates into the URL only when you press it.
