Use our pregnancy conception calculator to estimate when you conceived from a due date, last period, or ultrasound, with possible date ranges. Calculate now.
Choose the pregnancy date you know to estimate conception, a possible intercourse window, gestational age, and your due date.
Dates are estimates for general information. They are not a diagnosis and do not replace dating by your prenatal or fertility care team.
Start with the pregnancy date you know, understand why the answer is a range, and connect that estimate to the rest of your pregnancy calendar.
A conception date estimate can begin with an established pregnancy due date, the first day of your last menstrual period, or an ultrasound date with the gestational age reported at that scan. Choose the record you can identify most confidently rather than trying to reconstruct a date from symptoms alone.
Each input follows a different path to the same calendar estimate. A due date works backward by about 266 days, an LMP estimate adjusts for average cycle length, and ultrasound dating first finds an equivalent LMP. The calculator never reads ultrasound images or replaces the date documented by your prenatal care professional.
Compare the calculation methods
Calendar formulas produce a central estimated conception date, but ovulation does not always occur on the same cycle day. Fertilization timing, sperm survival, and the uncertainty already present in a due date or ultrasound estimate can move the biologically possible window in either direction.
That is why the result separates a narrower estimated conception range from a broader possible intercourse window. The dates help organize what may have happened; they cannot identify one act of intercourse, establish paternity, or confirm the exact day fertilization occurred.
Estimate ovulation and fertile days
Once you have an estimated conception date, you can place it within the standard pregnancy dating system. Gestational age starts about two weeks before conception in a typical 28-day cycle model, while an estimated due date falls about 266 days after conception.
Use those dates to orient yourself to pregnancy weeks, trimesters, and general milestone dates. They are planning references rather than appointment instructions or delivery predictions. A qualified care professional should determine which dates guide prenatal testing and clinical decisions.
Build your pregnancy timeline
An LMP-based estimate and an ultrasound-based estimate can differ when cycle timing varies, the menstrual date is uncertain, or the scan was performed later in pregnancy. An online calculator can show how each method translates into dates, but it cannot decide which record is clinically correct.
If your results disagree with a date in your prenatal record, keep the documented due date and ask your care team how it was established. Early ultrasound dating and assisted-reproduction records may carry different clinical weight than a calendar estimate, depending on your circumstances.
Compare pregnancy due date methodsA standard pregnancy due date is 280 days, or 40 weeks, from the first day of the last menstrual period. In a 28-day cycle model, conception occurs about 14 days after that starting point. The calculator therefore subtracts 266 days, or 38 weeks, from an entered due date to produce a central estimated conception date.
That central date is a calendar estimate, not proof of the exact day fertilization happened. Pregnancy length varies, and a due date may have been set from menstrual dates, ultrasound measurements, or assisted-reproduction records. Use the due date documented by your prenatal care professional when working backward.
When you calculate conception from the first day of your last menstrual period, the tool estimates ovulation about 14 days before the next expected period. A 28-day cycle places that estimate near cycle day 15 when the first day of bleeding is counted as day one. Longer cycles shift the estimate later, while shorter cycles shift it earlier.
The method works best as a broad guide when cycles are reasonably regular and the LMP date is known. If cycle lengths change from month to month, postpartum bleeding is mistaken for a period, or hormonal treatment affected the cycle, the entered average may not represent the cycle in which pregnancy began.
For the ultrasound method, enter the scan date and the gestational age reported on that date. The calculator subtracts the reported weeks and days to estimate an equivalent LMP, then adds about two weeks to estimate conception. It does not interpret crown-rump length, images, fetal growth, or the clinical quality of the scan.
ACOG states that first-trimester ultrasound is the most accurate ultrasound method for establishing or confirming gestational age. Later scans have wider dating uncertainty, so a later measurement should not be used here to overrule an established clinical due date without guidance from the care team.
People often use conception and fertilization to mean the same event, but they are not always defined identically. This calculator uses estimated ovulation and fertilization timing as the practical conception date. Implantation occurs later and cannot be calculated from a due date, LMP, or ultrasound with day-level certainty.
A positive pregnancy test also cannot identify the day conception occurred. Test timing depends on implantation and rising hormone levels as well as test sensitivity. The result is most useful for understanding a likely part of the calendar, not reconstructing a single confirmed event.
Sperm may remain capable of fertilization for several days before ovulation, while the egg is available for a much shorter time after ovulation. That is why intercourse that led to pregnancy may have happened before the central estimated conception date. The calculator displays a broader possible intercourse window alongside the narrower conception range.
Neither range can establish paternity or distinguish between partners when dates overlap. Dating formulas do not analyze DNA and cannot account for every variation in ovulation, sperm survival, fertilization, or implantation. Questions with medical or legal consequences require the appropriate qualified professional and, where relevant, valid testing.
Our formulas and interpretation notes were reviewed against the following clinical and public-health references on August 2, 2026. These sources explain general dating methods and do not validate an individual result.
No. It estimates a central date from the information entered. Variation in ovulation, fertilization, ultrasound dating, and pregnancy length means calendar arithmetic usually cannot confirm one exact day.
Subtract 266 days, or 38 weeks, from the due date for a central estimate. This calculator also shows a surrounding conception range because the due date itself is an estimate.
In standard obstetric dating, conception occurs at about 2 weeks 0 days of gestational age. Pregnancy weeks are counted from an equivalent LMP date rather than from fertilization.
An early ultrasound can establish or confirm gestational age more accurately than a later scan, but it still estimates pregnancy age rather than observing the exact moment of conception.
No. It can show a biologically possible range, but overlapping dates and natural variation prevent it from identifying a specific act of intercourse or determining paternity.
Conception Calculator