How the Pregnancy Calculator Works
The Pregnancy Calculator estimates your due date and builds a week-by-week timeline of your pregnancy. You choose the date you know, and it works out the other key dates, the week you are in now, the trimester, the average size of the baby, and a table of all 42 weeks with milestones.
Three ways to calculate
- Last Period Date: the first day of your last menstrual period (LMP). This is the most common starting point.
- Conception Date: the day the baby was conceived, if you know it.
- Due Date: a due date you already have (for example from a scan), to see the rest of the timeline.
Choose the method, pick the date, and press Calculate Timeline. The button "Use today's date" fills in today's date for you.
How the dates are worked out
Doctors count a pregnancy from the first day of the last period, not from conception. A full pregnancy is 40 weeks, which is 280 days.
Due date = last period date + 280 days Conception (approx.) = last period date + 14 days Last period (from conception) = conception date - 14 days Due date (from conception) = conception date + 266 days
The calculator counts whole calendar days, so a clock change does not move a date by a day.
A worked example
The first day of your last period was 1 January 2027:
| Item | Date |
|---|---|
| First day of last period | Jan 1, 2027 |
| Likely conception (+14 days) | Jan 15, 2027 |
| Estimated due date (+280 days) | Oct 8, 2027 |
Weeks and trimesters
The week is counted by completed weeks, and the trimester follows the completed weeks:
| Trimester | Completed weeks |
|---|---|
| First | 0 to 12 |
| Second | 13 to 27 |
| Third | 28 and over |
If the date you chose is in the future, the calculator tells you how many days until the pregnancy starts. If it is more than 42 weeks ago, it shows the pregnancy as past the due date, with the real number of weeks.
Average baby size
From week 8 the calculator shows an average size for the week. The numbers are averages, and every baby is different. Up to week 19 the length is crown-to-rump (head to bottom). From week 20 it is crown-to-heel (head to foot), which is why the length jumps at week 20.
| Week | Length | Weight |
|---|---|---|
| 12 | 5.4 cm | 14 g |
| 16 | 11.6 cm | 100 g |
| 20 | 25.6 cm | 300 g |
| 24 | 30.0 cm | 600 g |
| 28 | 37.6 cm | 1,005 g |
| 32 | 42.4 cm | 1,702 g |
| 36 | 47.4 cm | 2,622 g |
| 40 | 51.2 cm | 3,462 g |
Chart: average weight by week
Most of the weight is gained in the last trimester:
Week 12 ▏ 14 g Week 16 █ 100 g Week 20 ███ 300 g Week 24 ███████ 600 g Week 28 ████████████ 1,005 g Week 32 ████████████████████ 1,702 g Week 36 ██████████████████████████████ 2,622 g Week 40 ████████████████████████████████████████ 3,462 g
Chart: average length by week
Week 12 ████ 5.4 cm Week 16 █████████ 11.6 cm Week 20 ████████████████████ 25.6 cm Week 24 ███████████████████████ 30.0 cm Week 28 █████████████████████████████ 37.6 cm Week 32 █████████████████████████████████ 42.4 cm Week 36 █████████████████████████████████████ 47.4 cm Week 40 ████████████████████████████████████████ 51.2 cm
The week-by-week table
Below the result is a table of weeks 1 to 42 with the date range for each week, the trimester and some milestones. The current week is marked "Today". The milestones include when the heartbeat can be seen on an ultrasound (weeks 6 to 7), when the risk of miscarriage falls (week 13), when movements are felt (weeks 18 to 21), when a premature baby may survive (week 23), and the terms early term (weeks 38 and 39), full term (weeks 40 and 41) and late term (week 42 and over).
Good to know
- Only about 5% of babies arrive on the due date. A normal birth can happen from about 37 to 42 weeks.
- The calculation assumes a 28-day cycle. If your cycle is shorter or longer, or you are not sure of the date, an early ultrasound gives a better estimate.
- The ranges are averages, and a baby that is smaller or larger than the table is usually normal.
This tool gives an estimate for general information only. It is not medical advice. Always follow your doctor's or midwife's dates and care.