What this calculator does
A pregnancy countdown calculator answers the question that comes up most once a due date is known: how long is left. Enter the due date directly, or the first day of the last period if a due date has not been given yet, and it counts down in days and weeks, and estimates roughly how far through the pregnancy that puts you today.
This is a different reading from a gestational age calculator, which reports how far along a pregnancy is by counting forward from the last period. This one counts backward from the due date instead, because "how many weeks pregnant am I" and "how many days until my due date" are both useful but they are not the same number, and full-term babies can arrive anywhere from about three weeks early to two weeks after the estimated date.
The formula
If a due date is entered directly, the countdown is simply that date minus the reference date. If the first day of the last period is entered instead, the estimated due date is calculated first as 280 days (40 weeks) after that date, using the standard obstetric estimate, and the countdown runs from there. The percentage through the pregnancy assumes the same 280-day span, and the approximate conception date is shown as roughly 266 days before the due date, allowing for the usual two extra weeks counted from the last period rather than from conception itself.
| Term | Meaning |
|---|---|
| Due date | The estimated date of birth, either entered directly or calculated as 280 days after the last period. |
| Days remaining | Due date minus the reference date, shown in both total days and weeks-plus-days. |
| Estimated conception date | Roughly 266 days before the due date, since the standard 280-day estimate is counted from the last period, about two weeks before conception typically occurs. |
The inputs explained
| Field | What to enter |
|---|---|
| What you know | Choose whichever date you actually have: a due date already given by a scan or midwife, or the first day of the last menstrual period if no due date has been set yet. |
| Due date (due-date mode) | The estimated due date, as given by a dating scan or care provider. |
| First day of last period (last-period mode) | The first day of the last menstrual period, used to estimate a due date 280 days later. |
| Count down from | The date to count down from. Defaults to today. |
When to use it
Planning around a known due date
Once a scan has set a due date, the countdown turns it into a concrete number of weeks and days left, which is easier to plan leave, travel or a hospital bag around than a calendar date on its own.
Estimating before a dating scan
Early on, before an ultrasound has confirmed a due date, the last-period estimate gives a rough countdown to work from, with the understanding that a scan may adjust it once dating is more precise.
Checking how far through things are
The percentage-through figure gives a quick sense of overall progress through the pregnancy, alongside the trimester detail a gestational age calculator provides.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the countdown looks at different points in a pregnancy
Using a fixed due date, the countdown from a range of earlier reference dates.
| Counting down from | Time remaining | Roughly through the pregnancy |
|---|---|---|
| 1/9/2026 | 12 weeks, 5 days | 68.2% |
| 29/9/2026 | 8 weeks, 5 days | 78.2% |
| 27/10/2026 | 4 weeks, 5 days | 88.2% |
| 10/11/2026 | 2 weeks, 5 days | 93.2% |
| 22/11/2026 | 1 weeks, 0 days | 97.5% |
| 28/11/2026 | 0 weeks, 1 days | 99.6% |
Questions
Is the due date exact?
No. It is a statistical estimate, and most births happen within a window of roughly three weeks either side of it. Treat the countdown as a planning guide rather than a guaranteed date, and follow whatever date a dating scan has given if it differs from a last-period estimate.
Why does a scan sometimes move the due date?
A dating scan measures the baby directly, which is usually more accurate than counting from the last period, particularly if cycles are irregular or the exact start date of the last period is uncertain. When a scan gives a different due date, that is normally the one to use.
What if the due date has already passed?
The calculator switches to showing time since the due date instead of time remaining. Going somewhat past the estimated date is common and is not on its own a cause for concern, but follow the guidance given by your care provider.
How is the percentage through the pregnancy worked out?
It treats the pregnancy as a fixed 280-day span from the last period to the due date, and works out how far the reference date sits along that span. It is a rough guide alongside the day and week count, not a clinical measurement.
To see gestational age in weeks and trimester detail counted forward from the last period instead, use the gestational age calculator. For a plain days-and-weeks count to any other date, see the countdown to a date calculator.