Pregnancy Calculator - Due Date Estimate

The Pregnancy Calculator estimates a due date and how many weeks along a pregnancy is, using standard obstetric dating methods. Enter a last menstrual period date, a conception date, an ultrasound dating point, or an IVF transfer date, and the tool returns an estimated due date plus current gestational age and trimester when a reference "today" is known.

A due date is a planning marker, not a deadline stamped on a calendar with certainty. Most births do not occur on the estimated day. This page shows how each dating method works, walks the arithmetic, and keeps the tone factual and open to many reasons someone might be checking the numbers.

Calculate your due date and how far along you are

Gestational age in clinical practice is usually counted from the first day of the last menstrual period (LMP), which places "week zero" about two weeks before ovulation in an average cycle. From that framework, a full-term estimate is 40 weeks, or 280 days, after LMP.

The calculator accepts several starting points because real records differ. Some people know LMP clearly; others have an early ultrasound report; others used IVF and know the transfer date exactly. Pick the method that matches the information on hand. The output includes an estimated due date and, when useful, the current week and trimester.

Figures here are estimates for general information. They do not replace prenatal care, ultrasound dating in clinic, or advice from a midwife or obstetric clinician. Pregnancies vary; this page does not assume why the date is being checked or how the pregnancy will proceed.

Calculate from your last menstrual period

Naegele's rule is the classic LMP method. Add 280 days (40 weeks) to the first day of the last menstrual period. An equivalent memory aid used in some teaching settings is: subtract three months, add seven days, and adjust the year.

Due date ≈ LMP + 280 days

Source convention: Naegele's rule; ACOG pregnancy dating practices use the same 280-day framework from LMP when LMP dating is appropriate.

LMP dating assumes a roughly 28-day cycle with ovulation near day 14. Shorter or longer cycles shift the true ovulation day, which is why an early ultrasound often revises the estimate. If cycles are irregular, treat the LMP result as a first sketch and prefer scan-based dating when available.

Enter the first day of bleeding for that period, not the mid-period date and not the day of a positive test. Small input errors move the due date by the same number of days.

Calculate from your conception date

Some people know a conception or transfer-related date more clearly than an LMP. When that conception date is known with reasonable confidence, gestational length from conception is about 266 days, or 38 weeks, in the reference used on this site.

Due date ≈ conception date + 266 days

This path is useful when LMP is unknown or unreliable but a conception date was recorded. It is still an estimate: fertilisation timing and implantation are not stopwatch events even when intercourse or insemination timing is known.

In average cycles, conception sits about 14 days after LMP, which is why 280 days from LMP and 266 days from conception describe the same span. The calculator keeps both methods so the input can match the fact pattern rather than forcing every user through LMP.

Calculate from an ultrasound date

First-trimester ultrasound dating is often more accurate than LMP when cycles are irregular or the LMP date is uncertain. The clinic reports gestational age at the scan; the calculator projects forward to a 280-day total from the implied LMP-equivalent timeline.

Due date ≈ scan date + (280 - gestational age in days at scan)

Example idea: a scan at exactly 7 weeks 0 days (49 days) leaves 231 days until the 280-day mark, so due date is scan date plus 231 days. Real reports use weeks and days; convert to total days before subtracting from 280.

Later scans can still estimate age but with wider uncertainty. When an early scan and LMP disagree by a clinically meaningful margin, obstetric teams usually adopt the scan. This tool shows the ultrasound-based estimate; the care team decides which date chart to follow.

Calculate from an IVF transfer date

IVF dating uses the documented transfer date and the age of the embryo at transfer. Clinics record that age in days. The reference span from a day-0 conception equivalent is 266 days, so embryo age is subtracted from that span before adding to the transfer date.

Due date ≈ transfer date + 266 - embryo age in days

For a day-5 blastocyst transfer, embryo age is 5 days, so add 261 days to the transfer date. For a day-3 transfer, add 263 days. Exact clinic worksheets may use the same arithmetic under slightly different labels; match the embryo age to what the clinic recorded.

IVF dating is often the most precise of the self-reported methods because the transfer moment is documented. It remains an estimate of birth timing. The calculator does not adjust for frozen versus fresh beyond the embryo-age input you provide.

Find your current week and trimester

Once a due date or LMP anchors the timeline, current gestational age is how far along the pregnancy is relative to the usual 40-week framework. The calculator can show the present week when a reference "today" is available. Trimesters in the reference used here are:

Trimester	Weeks
First	0 to 13
Second	14 to 27
Third	28 to 40+

The calculator reports the current week when a reference date (today, or a date you enter) is available. Week numbering can confuse because "week 12" means the pregnancy has completed 11 full weeks and is in the 12th week; clinic apps sometimes display that differently. If a number on this page and a number on a clinic printout differ by a day or two, ask the care team which chart is official.

Trimester labels are organisational, not grades of success. They simply group weeks for education and appointment planning.

Understand that a due date is an estimate

Only a small share of births happen on the estimated due date. A commonly cited figure is that roughly 1 in 20 births occur on that exact day; most occur within about two weeks either side. Induction, scheduled caesarean birth, and spontaneous timing all affect when labour starts relative to the estimate.

Calling the date an estimate is not a soft apology; it is how obstetric dating works. The number organises prenatal schedules and gives a shared reference. It does not guarantee a birth day, and it does not define the worth or outcome of a pregnancy.

This page stays neutral about the pregnancy's course and about why someone is using the tool. Some users are planning; some are comparing clinic dates; some are checking after a loss or during a complicated path. The arithmetic is the same. Care decisions belong with qualified clinicians who know the full history.

Keep clinic dates as the official record

Self-serve calculators help with understanding and with double-checking arithmetic. The dating chart on a prenatal record remains the schedule clinics use for appointments and screening windows. If this page and a clinic printout disagree, ask which date the care team wants on the chart and why; early ultrasound revisions are a common reason for a shift of several days.

Bring questions about symptoms, screening choices and birth planning to the midwife or obstetric clinician. Dating math alone cannot answer those. The Pregnancy Calculator stays limited to estimate arithmetic on purpose.

Frequently asked questions

Summary

The Pregnancy Calculator estimates due date and gestational timing from LMP (plus 280 days), conception (plus 266 days), ultrasound projection to a 280-day span, or IVF transfer dating (266 days minus embryo age). It reports trimester bands (0 to 13, 14 to 27, 28 to 40+) when current week is available.

A due date is an estimate: most births are not on that exact day. The tone stays inclusive and factual. Results are general information, not a substitute for clinical prenatal care.