This due date calculator, provided by Hesapstan, turns the first day of your last period into an estimated due date, your completed weeks and days of pregnancy today, the days remaining (or days past the estimated date) and the calendar dates of the 37, 39, 40, 41 and 42-week milestones (the ACOG term thresholds plus the estimated due date). The standard rule is first day of the last period + 280 days; if your cycle is regular but not 28 days, an optional cycle-length correction can be applied. The result is an estimate: it does not replace a date set by your healthcare professional or ultrasound findings.
What does this calculator work out?
It works from one date: the first day of your last period. You type the four-digit year yourself; the date is never silently guessed or corrected. When you press Calculate you see:
- The estimated due date (with the weekday) and the day the calculation was made
- Completed weeks and days of pregnancy (for example 10 weeks 1 day), today’s trimester and, where it applies, the ACOG term status
- Days remaining; “today” if the date is today, or how many days past the estimated date if it has passed
- How much of the 280-day pregnancy calendar has passed (day counts and percentages)
- Calendar dates for the 37, 39, 40, 41 and 42-week milestones
- How the date was calculated, its limits and the sources
It does not date a pregnancy from an ultrasound measurement, an IVF transfer or a date given by your clinician; for those use the advanced Pregnancy Calculator on Hesapstan. It does not confirm that a pregnancy exists and does not diagnose anything.
How is the due date calculated?
Estimated due date = first day of the last period + 280 days (40 weeks). This is the standard rule used by ACOG (Committee Opinion 700) and the NHS, and it assumes a 28-day cycle.
If you have a regular cycle that is not 28 days, you can enter its length as a whole number from 21 to 45. The calculator applies the conventional correction: estimated due date = first day of the last period + 280 + (cycle length − 28) days. For example, a 32-day cycle moves the date 4 days later and a 25-day cycle 3 days earlier. If you leave it blank, 28 days is assumed.
If your cycle is irregular this correction is not reliable and a date from your last period stays uncertain. Dating by an early (first-trimester) ultrasound, or together with your healthcare professional, is more reliable.
How to read the week and day
Gestational age is given as completed weeks and days since the first day of the last period. “10 weeks 1 day” means 71 days have passed since that day. For that reason, with a 28-day cycle, the first 14 days or so come before fertilization.
Some sites answer “which week of pregnancy are you in?” with an ordinal number and show 10 weeks 1 day as “week 11”. Both describe the same day; only the naming differs. This calculator uses the clinical convention of completed weeks plus days, so 10 weeks 0 days and 10 weeks 1 day are never confused.
- First trimester: up to 13 weeks 6 days
- Second trimester: 14 weeks 0 days – 27 weeks 6 days
- Third trimester: from 28 weeks 0 days
Term milestones: 37, 39, 40, 41 and 42 weeks
ACOG (Committee Opinion 579) names births from 37 weeks 0 days in four groups: early term (37 weeks 0 days – 38 weeks 6 days), full term (39 weeks 0 days – 40 weeks 6 days), late term (41 weeks 0 days – 41 weeks 6 days) and postterm (42 weeks 0 days and beyond). For each threshold the calculator shows the calendar date for your dates and whether it is in the past, today or ahead.
The milestones do not say when birth will or should happen; they are definitions that name the week in which a birth takes place. The timing of birth is decided with your care team according to you and your situation.
What does it mean if the estimated date has passed?
Many babies are born before or after the estimated date; it is not a certain birthday. Once the date has passed, the calculator replaces “days remaining” and the progress bar with a plain statement of how many days it is past the estimated date, and keeps showing the other results.
- 41 weeks 0 days – 41 weeks 6 days (late term): if the pregnancy is still ongoing, discuss the follow-up and monitoring plan with your maternity-care professional.
- 42 weeks 0 days and beyond (postterm): if the pregnancy is still ongoing, contact your maternity-care team promptly.
- A gestational age beyond 45 weeks 0 days is not accepted; the year or date was very probably mistyped and no result is shown.
The tool only counts forward on the calendar from the date you entered. It cannot know whether a pregnancy is ongoing. The advice to contact your care team at 41 and 42 weeks is general safety guidance; the tool does not diagnose and does not recommend individual treatment or a timing of birth. Assessment and the timing of birth are decided individually with your care team.
Development and exposures: why there is no “risk window”
How an exposure such as a medicine, an infection or radiation can affect a developing baby depends on the organ or system involved, the kind and amount of exposure and its timing. An exposure alone does not mean that harm has occurred. Many organ systems form early, so early pregnancy is particularly sensitive for structural development; some effects can still matter later, for example on growth or the nervous system.
For that reason we do not show a fixed “high-risk interval” derived from the last-period date: such an interval is not a personal risk calculation and can cause needless alarm. Some sources count weeks from conception, which is about two weeks behind the gestational age shown here. For a specific exposure, talk to your clinician, your pharmacist or your local teratology information service (for example MotherToBaby in North America); do not stop a prescribed medicine without medical advice.
If you do not know the date or have other information
If you have no last-period date, your cycle is irregular, or you have an early ultrasound or a date given by your clinician, that information is the more reliable route. The advanced Pregnancy Calculator on Hesapstan accepts a due date given by your doctor or midwife, an ultrasound report, an early scan (CRL) measurement, an IVF embryo transfer date and a documented fertilization or laboratory insemination date.
The date of intercourse and the date of a positive pregnancy test are not fertilization dates: sperm can survive for several days and fertilization can happen after intercourse. We therefore do not turn those dates into an automatic “+266 days” estimate.
How to use the calculator
- Choose or type the first day of your last period as day, month and a four-digit year.
- If you have a regular cycle that is not 28 days, enter its length (21–45); leave it blank if you are unsure.
- Press “Calculate”.
- Read the estimated due date, the completed weeks and days, the days remaining, the calendar progress and the 37–42 week dates.
- Changing any value or pressing “Reset” clears the previous result; calculate again for a new one.
Examples
These are the dates the calculator produces; “today” is taken as 10 October 2026.
Example 1: last period 1 August 2026, cycle blank (28 days)
- Estimated due date: Saturday 8 May 2027 (1 August 2026 + 280 days)
- On 10 October 2026: 10 weeks 0 days, 70 days elapsed, 210 remaining (25% / 75%)
- Milestones: 37 weeks 0 days is 17 April 2027, 39 weeks 0 days is 1 May 2027, 41 weeks 0 days is 15 May 2027, 42 weeks 0 days is 22 May 2027
Example 2: same date, cycle 32 days
The correction is (32 − 28) = +4 days, so the estimated due date is Wednesday 12 May 2027. On 10 October 2026 the gestational age is 9 weeks 3 days: 66 days elapsed and 214 remaining (24% / 76%). Because the days are counted from the corrected date rather than the date you typed, you never see an inconsistent total such as 70 + 214; elapsed plus remaining is always 280.
Common mistakes
- Entering the day the period ended instead of the first day it started
- Entering the date of intercourse, a late period or a positive test instead of the last-period date
- Mistyping the year; results beyond 45 weeks are refused for that reason
- Relying on the cycle correction when the cycle is irregular
- Reading the estimated date as a certain birthday
Frequently Asked Questions
How is the due date calculated?
The estimated due date is the first day of the last period plus 280 days (40 weeks). If you have a regular cycle that is not 28 days, the calculator applies a (cycle − 28) day correction. The result is an estimate; a date set by your healthcare professional takes precedence.
What if I do not know the date of my last period?
A date from your doctor or midwife, or an early (first-trimester) ultrasound, is more reliable. To calculate from that information, use the advanced Pregnancy Calculator on Hesapstan; this tool works only from the last-period date.
Can a due date be calculated from intercourse or a positive test date?
Not with this tool. The date of intercourse and the date of a positive test are not fertilization dates; sperm can survive for several days and fertilization can happen after intercourse. If you have a documented fertilization or laboratory insemination date, the advanced Pregnancy Calculator can use it.
How does cycle length change the result?
The standard calculation assumes a 28-day cycle. If you enter a regular cycle of 21–45 days the date shifts by (cycle − 28) days: 4 days later for a 32-day cycle, 3 days earlier for a 25-day cycle. If your cycle is irregular this correction is not reliable.
Why does it say “10 weeks 1 day” instead of “week 11”?
Because gestational age is given clinically as completed weeks and days. Some sites write the same day as the ordinal “week 11”. Both describe the same day; this tool shows weeks and days together to avoid ambiguity.
What should I do if my estimated due date has passed?
Many babies are born after the estimated date. If the pregnancy is still ongoing and you have reached 41 weeks, discuss the follow-up plan with your care team; if it is still ongoing at 42 weeks 0 days or beyond, contact them promptly. The calculator only counts calendar days and gives no individual medical direction.
Why is no “high-risk period” shown?
The effect of an exposure depends on the organ, the kind and amount of exposure and its timing; a fixed interval derived from the last-period date is not a personal risk calculation. We therefore give neither such an interval nor a risk score. For a specific exposure, ask your clinician or your local teratology information service (for example MotherToBaby in North America).
Will the birth happen exactly on the estimated date?
No. The estimated date is an average; most babies are born before or after it. The term thresholds (37, 39, 41 and 42 weeks) are ACOG definitions that name the week in which a birth takes place; 40 weeks is the estimated due date. They are not a delivery schedule.
Why does it refuse some dates?
Dates that do not exist (for example 31 April), dates after today, years outside 1900–2100, cycle lengths outside 21–45 or not whole numbers, corrected dates that imply the pregnancy has not started, and gestational ages beyond 45 weeks 0 days are refused. In each case a message states the reason; no result is invented.
Does this calculation show that a pregnancy exists?
No. The tool only counts forward on the calendar from the date you enter. Even a last-period date of today returns 0 weeks 0 days; that does not show or confirm a pregnancy.