How pregnancy is counted
Pregnancy is dated from the first day of the last menstrual period (LMP), not from conception. That convention — gestational age — puts you at four weeks pregnant on the day a period is missed, even though the embryo is about two weeks old. A full-term pregnancy is 40 weeks, or 280 days, from the LMP; conception happens around day 14. Every dating method this calculator accepts is converted to an LMP-equivalent start date, and the whole calendar runs from there, which keeps weeks, trimesters and the due date consistent whichever date you know.
The table shows the dated calendar for a last period on January 5, 2026. Week N runs from the day you turn N weeks (N weeks 0 days) to N weeks 6 days.
| Week | Starts | Ends | Trimester |
|---|---|---|---|
| 4 | February 2, 2026 | February 8, 2026 | 1 |
| 8 | March 2, 2026 | March 8, 2026 | 1 |
| 12 | March 30, 2026 | April 5, 2026 | 1 |
| 13 | April 6, 2026 | April 12, 2026 | 1 |
| 20 | May 25, 2026 | May 31, 2026 | 2 |
| 24 | June 22, 2026 | June 28, 2026 | 2 |
| 27 | July 13, 2026 | July 19, 2026 | 2 |
| 28 | July 20, 2026 | July 26, 2026 | 3 |
| 32 | August 17, 2026 | August 23, 2026 | 3 |
| 36 | September 14, 2026 | September 20, 2026 | 3 |
| 37 | September 21, 2026 | September 27, 2026 | 3 |
| 39 | October 5, 2026 | October 11, 2026 | 3 |
| 40 | October 12, 2026 | October 18, 2026 | 3 |
| 41 | October 19, 2026 | October 25, 2026 | 3 |
| 42 | October 26, 2026 | November 1, 2026 | 3 |
Dating from a different starting point
If you know the conception date — from ovulation tracking or fertility treatment — the due date is 266 days later. For IVF the transfer date plus the embryo's age at transfer fixes conception exactly, which makes IVF pregnancies the most precisely dated of all. An ultrasound report gives a gestational age on the scan date, from which the LMP-equivalent start is back-calculated. A cycle longer or shorter than 28 days shifts ovulation and therefore the due date by the same number of days.
| Known date | Value | Rule | Due date |
|---|---|---|---|
| Last period (28-day cycle) | January 5, 2026 | + 280 days | October 12, 2026 |
| Last period, 32-day cycle | January 5, 2026 | + 284 days | October 16, 2026 |
| Conception | January 19, 2026 | + 266 days | October 12, 2026 |
| IVF day-5 transfer | January 24, 2026 | + 261 days | October 12, 2026 |
| Ultrasound at 8w 0d | March 2, 2026 | − 56 days, + 280 | October 12, 2026 |
Trimesters, months and weeks
The first trimester runs to the end of week 13, the second to the end of week 27 and the third to birth; some sources put the boundaries a week earlier or later, and the calculator uses the 13/27 split adopted by the American College of Obstetricians and Gynecologists. 'Months' of pregnancy are looser: 40 weeks is a little over nine calendar months, so the common mapping gives some months four weeks and others five.
| Month | Weeks | Trimester |
|---|---|---|
| Month 1 | Weeks 1–4 | 1 |
| Month 2 | Weeks 5–8 | 1 |
| Month 3 | Weeks 9–13 | 1 |
| Month 4 | Weeks 14–17 | 2 |
| Month 5 | Weeks 18–22 | 2 |
| Month 6 | Weeks 23–27 | 2 |
| Month 7 | Weeks 28–31 | 3 |
| Month 8 | Weeks 32–35 | 3 |
| Month 9 | Weeks 36–40 | 3 |
Term, preterm and post-term
Only about 4% of babies arrive on the due date; most come within two weeks either side. ACOG divides the weeks around the due date into early, full and late term because outcomes differ measurably between them, which is why elective deliveries are scheduled from 39 weeks and induction is usually offered by 41–42 weeks.
| Category | Gestational age | Dates | Share of births |
|---|---|---|---|
| Preterm | before 37w 0d | before September 21, 2026 | About 10% of US births |
| Early term | 37w 0d – 38w 6d | September 21, 2026 – October 4, 2026 | About 26% |
| Full term | 39w 0d – 40w 6d | October 5, 2026 – October 18, 2026 | About 57% |
| Late term | 41w 0d – 41w 6d | October 19, 2026 – October 25, 2026 | About 6% |
| Post-term | 42w 0d and after | from October 26, 2026 | Under 1%; induction usual |
Milestones in the calendar
The week-by-week table on this page marks the events most prenatal schedules share: the first visit and dating scan around 8 weeks, cell-free DNA screening from 10 weeks, the nuchal translucency scan at 11–14 weeks, the anatomy scan at 18–22 weeks, glucose screening at 24–28 weeks, the Tdap vaccine and Rh antibody check in the third trimester, group B strep testing at 35–37 weeks and weekly visits from 36. Timing varies by provider and country; treat the table as a guide to what to expect, not a schedule.
- Weeks 1–4: conception and implantation; a test turns positive around the missed period.
- Weeks 5–13: first trimester; organ formation; dating scan; miscarriage risk falls sharply by week 13.
- Weeks 14–27: second trimester; anatomy scan; movements felt from about 18–20 weeks; viability from 24.
- Weeks 28–40: third trimester; growth and positioning; weekly visits from 36; full term from 39.
How accurate is the dating?
Period-based dating assumes ovulation on day 14 and accurate recall of the LMP; either can be off by several days. A first-trimester ultrasound measures the embryo directly and dates the pregnancy to within about five days, so when it disagrees with the LMP date by more than that, clinicians adopt the scan date. Later scans are less precise because babies grow at increasingly different rates. If your provider has given you a due date, choose 'Known due date' and enter it; the calendar will match your chart.