Health Estimate
Pregnancy Calculator
Estimate your due date, current pregnancy week, trimester, conception window, and milestone dates. Choose the dating method that matches the information you actually have.
How This Pregnancy Calculator Works
This Calculator+ tool converts the dating method you choose into one estimated due date, then builds gestational age, trimester, conception-window, and milestone outputs from that anchor. The logic changes depending on the input method because not all pregnancy dates come from the same evidence quality.
LMP dating uses 280 days from the first day of the last menstrual period, with an adjustment for your usual cycle length. Conception dating uses 266 days from conception. IVF dating uses the transfer date plus the embryo-age rule rather than guessing ovulation timing. Ultrasound dating works from the gestational age already reported on the scan date and adds the remaining days to 40 weeks.
The page is therefore not just a 40-week countdown. It is a date-conversion tool that tries to match the kind of information people actually have after a positive test, fertility treatment, or early prenatal appointment. It still remains an estimate, which is why the safety note directs users back to their maternity team for confirmed dating and care decisions.
Pregnancy Dating Notes
Pregnancy dating is usually described in weeks and days from the first day of the last menstrual period, even though conception usually happens later. That convention is why someone can be described as “4 weeks pregnant” before four weeks have passed since conception itself. The language is standard in obstetrics, but it is often confusing to first-time users.
A 40-week due date is also not an appointment. NHS guidance notes that pregnancy normally lasts from 37 to 42 weeks from the first day of the last menstrual period, and ACOG treats the estimated due date as exactly that: an estimate. Many pregnancies deliver before or after the calculated day, and clinical circumstances can change timing further.
Clinical dating can change after review of early ultrasound, cycle history, fertility-treatment records, and medical context. If your clinician gives you a different due date, use that confirmed date for medical planning. The web tool is most useful for understanding the timeline, not for overruling a documented prenatal dating decision.
Last Menstrual Period Dating and Cycle Length
The LMP method is the most familiar due-date rule because it is simple and widely used: start from the first day of the last period and count forward 280 days. The problem is that this assumes a regular 28-day cycle with ovulation around day 14. Real cycles are often not that tidy.
That is why this page lets you enter a usual cycle length instead of forcing everyone into the 28-day assumption. A longer cycle can push ovulation later, while a shorter cycle can bring it earlier. This does not make the result exact, but it is more informative than acting as if all ovulation timing is identical.
Even with that adjustment, remembered period dates can still be uncertain. ACOG notes that many people do not recall the LMP precisely, and that is one reason early ultrasound often becomes the stronger dating reference when there is disagreement or uncertainty.
Why First-Trimester Ultrasound Often Matters Most
ACOG states that ultrasound measurement in the first trimester, up to and including 13 weeks 6 days, is the most accurate method to establish or confirm gestational age. Early crown-rump-length dating is more precise than later dating and is one of the main reasons prenatal due dates sometimes change after an early scan.
This matters because users often think the remembered period date should always win. In practice, if early ultrasound differs materially from LMP dating, clinicians may revise the estimated due date to match the ultrasound findings, especially when the LMP is uncertain or cycles are irregular.
That makes ultrasound one of the highest-value hidden variables on a due-date page. A person can enter a perfectly sincere LMP and still end up with a different clinical date later because the earlier biological measurement provided a better timeline anchor.
Conception Dating and What It Really Means
Conception dating is useful when the fertilization timing is unusually clear, but it is less common than LMP dating in general pregnancy discussions because most people do not know the exact conception date. When it is known, the usual due-date rule is 266 days from conception rather than 280 days from LMP.
That 14-day difference reflects the standard obstetric convention: gestational age usually starts from the period date, not from the actual day of fertilization. People often find that confusing because it feels like pregnancy is being dated “before conception,” but the convention is standard and intentional.
This page keeps conception as a separate method because some users genuinely have better conception timing than period timing, especially when they are tracking cycles closely. It is still an estimate, but it can be a better estimate than an uncertain LMP.
IVF Transfer Rules and Why They Differ
IVF dating is not just LMP dating with a different label. ACOG specifically notes that assisted-reproduction pregnancies should use the ART-derived gestational age to assign the estimated due date. That means the embryo age and the transfer date are the key inputs, not a guessed ovulation date from a menstrual cycle.
For example, the due-date offset from a day-5 embryo transfer is different from the offset for a day-3 transfer. This page exposes embryo age directly because that distinction materially changes the result. A page that collapses all IVF transfers into one generic due-date rule loses information that actually matters.
That is one reason IVF is a separate method instead of a footnote. Fertility-treatment records often provide more precise timing than a remembered period date, and the calculator should respect that rather than pretending every pregnancy starts from the same information quality.
Ultrasound Dating and Redating Limits
When you enter ultrasound dating here, you are effectively telling the calculator: “On this scan date, the pregnancy measured this many weeks and days.” The page then works forward to an estimated due date by adding the remaining time to 40 weeks. That makes it easy to translate a scan report into the familiar due-date framework.
Clinically, redating is not arbitrary. ACOG publishes guidance on when an ultrasound discrepancy is large enough to justify changing the estimated due date, and that threshold depends on how early the ultrasound was performed. Earlier scans are generally more accurate for dating than later ones.
This page does not try to replace those clinical thresholds. It simply lets the user work from an ultrasound-derived gestational age if that is the best information they have. If the ultrasound result conflicts with earlier assumptions, the prenatal care team should decide which date governs the medical record.
What the Milestone Timeline Can and Cannot Tell You
The milestone timeline is useful because it translates one estimated due date into practical date markers: current gestational age, trimester, conception window, full-term window, and other timeline checkpoints. That helps users orient themselves quickly after a positive test, an IVF transfer, or an early scan.
What it cannot do is predict labor onset, fetal growth quality, viability, or whether a pregnancy is developing normally. Those are medical questions that depend on symptoms, ultrasound findings, hormone trends, and clinical assessment. A due-date page should be explicit about that boundary.
The result is therefore best used as a planning and understanding tool. It helps users know where they are in the pregnancy timeline, but it does not replace appointments, scans, or professional review.
When to Rely on Your Clinician Instead of the Calculator
The calculator is most helpful when the pregnancy appears straightforward and the user wants a clearer view of dates. It becomes much less appropriate as a stand-alone tool when there is uncertain LMP, irregular cycles, IVF documentation, bleeding, pain, suspected ectopic pregnancy, multiple gestation, or an ultrasound result that does not match expected dates.
Those are not edge cases to brush aside. They are exactly the circumstances in which clinical confirmation matters most. A date estimate can still be interesting in those situations, but it should not be treated as the governing answer.
That is why the page keeps the interactive calculator above the fold but puts a strong medical note directly underneath. The tool can answer the timeline question quickly, but it should not create false confidence in situations that need real prenatal review.
Pregnancy Calculator FAQ
How does this pregnancy calculator estimate a due date?
It uses the selected dating method: 280 days from the first day of the last menstrual period, 266 days from conception, a transfer-date rule for IVF, a remaining-days rule for ultrasound dating, or a due date you already know.
Why does cycle length affect an LMP due date?
The common LMP method assumes a 28-day cycle with ovulation around day 14. A longer or shorter usual cycle can shift the estimate because ovulation may have happened later or earlier.
Which pregnancy dating method is most reliable?
Your clinician should confirm dating. Early ultrasound and assisted-reproduction dates can be more specific than a remembered period date, especially when cycles are irregular.
Is the due date the exact delivery date?
No. A due date is an estimate, not an appointment. Many pregnancies deliver before or after the estimated date, and medical circumstances can change timing.
Why can an early ultrasound change the due date?
Early ultrasound can date a pregnancy more accurately than a recalled period date, especially if cycles are irregular or the last menstrual period is uncertain. ACOG treats first-trimester ultrasound as the most accurate method to establish or confirm gestational age.
How is IVF dating different from standard LMP dating?
IVF dating uses the embryo age and the transfer date instead of assuming ovulation timing from a menstrual cycle. For example, a day-5 embryo transfer uses a different due-date offset than a day-3 transfer.
Can this calculator diagnose a pregnancy problem or viability issue?
No. This page estimates dates only. It cannot diagnose miscarriage, ectopic pregnancy, viability, labor timing, or fetal health.
Does a twin or multiple pregnancy change the estimated due date?
The standard dating rules still establish gestational age, but multiple pregnancy can change monitoring and delivery planning. Your maternity team should guide timing decisions in those cases.