Pregnancy is counted from the first day of the last period, about two weeks before conception, so a “40-week” pregnancy lasts about 38 weeks from conception. Only around 4–5% of babies arrive on their due date; most are born within two weeks either side. An early ultrasound (before 14 weeks) is the most accurate way to date a pregnancy.
How to use the Due Date Calculator
- Calculate from
- First day of your last period
- Average cycle length
- Conception date
- Embryo transfer date
- Embryo age at transfer
- Date of ultrasound
- Gestational age at the scan: weeks
- Gestational age at the scan: days
- Due date
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose what you know and enter the date. The calculator converts everything to an equivalent last-period date, then adds 280 days. Your progress is measured from that date to today.
Pregnancy milestones
| Week | Milestone |
|---|---|
| 4–5 | Positive pregnancy test |
| 6–7 | Heartbeat visible on ultrasound |
| 11–14 | Dating and nuchal translucency scan |
| 18–22 | Anatomy scan |
| 24 | Viability |
| 28 | Third trimester starts |
| 37–38 | Early term |
| 39–40 | Full term |
| 42 | Post term |
Inputs explained
| Field | What to enter |
|---|---|
| Calculate from | Choose one: Last menstrual period, Conception date, IVF transfer date, Ultrasound scan … |
| First day of your last period | A date |
| Average cycle length | In days |
| Conception date | A date |
| Embryo transfer date | A date |
| Embryo age at transfer | Choose one: Day 3 embryo, Day 5 blastocyst, Day 6 blastocyst |
| Date of ultrasound | A date |
| Gestational age at the scan: weeks | A number |
| Gestational age at the scan: days | A number |
| Due date | A date |
What each result means
- Estimated due date = [Equivalent LMP] + 280
- Estimated conception date = [Equivalent LMP] + 14
- Weeks pregnant today = floor([Days pregnant]÷7)
- …plus days = mod([Days pregnant], 7)
- Days until the due date = max([Equivalent LMP] + 280 – today(), 0)
- Second trimester starts = [Equivalent LMP] + 91
- Third trimester starts = [Equivalent LMP] + 189
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
From LMP: due date = LMP + 280 days + (cycle length − 28)
From conception: due = conception + 266 days
From IVF: due = transfer date + 266 − embryo age (days)
From ultrasound: due = scan date − gestational age at scan + 280 days
Worked example
Inputs: Calculate from: Last menstrual period; First day of your last period: 2026-08-16; Average cycle length: 28 days.
Results: Estimated due date = Sun, May 23, 2027; Estimated conception date = Sun, August 30, 2026; Weeks pregnant today = 12 weeks; …plus days = 1 days; Trimester = First trimester; Days until the due date = 195 days; Second trimester starts = Sun, November 15, 2026; Third trimester starts = Sun, February 21, 2027.
Key terms
- Naegele's rule
- Due date = first day of last period + 280 days (40 weeks).
- Gestational age
- Pregnancy length counted from the first day of the last period.
- Trimester
- One of three periods of pregnancy: weeks 1–13, 14–27 and 28 to birth.
Frequently asked questions
How accurate is a due date?
About 4% of babies are born on it; most arrive between 37 and 42 weeks.
What if my cycle is not 28 days?
The calculator shifts the due date by the difference between your cycle and 28 days.
Which method is best?
An early first-trimester ultrasound; if it differs from the LMP date by more than 7 days, the scan date is usually used.
When is a baby full term?
From 39 weeks 0 days to 40 weeks 6 days; 37–38 weeks is early term.
Sources
Guides on this topic
- How to Calculate Your Due Date: LMP, Conception, IVF and Ultrasound: Add 280 days to your last period, or 266 to conception. Here is how each method works, when an early scan changes the date, and what the trimesters look like.