Estimate a baby's due date from your last period, a known conception date, or an IVF transfer date.
Last period: adds 280 days (40 weeks) to the first day of your last menstrual period, adjusting slightly if your cycle isn't exactly 28 days.
Conception date: adds 266 days (38 weeks) to the date of conception, since pregnancy is typically dated from ovulation rather than the last period.
IVF transfer: counts forward from the transfer date, accounting for the embryo's age at transfer (3 or 5 days) to line up with the same 38-week-from-conception estimate.
Only about 5% of babies arrive on their exact due date — treat this as a planning estimate, and confirm dates with your doctor or midwife.
Choose the method that matches what you know: "Last period" if you know the first day of your last menstrual period, "Conception date" if you know or estimate when conception occurred, or "IVF transfer" if you conceived through IVF and know your embryo transfer date and embryo age at transfer. Enter the relevant date, and the calculator returns your estimated due date along with your current week of pregnancy, using the standard dating conventions each method relies on.
It might seem more logical to count pregnancy from the actual date of conception, but medical dating conventions use the first day of the last menstrual period (LMP) instead, largely because conception date is often unknown while the last period date is usually easy to identify. Since ovulation and conception typically occur about two weeks after the start of a period, "40 weeks from LMP" and "38 weeks from conception" describe roughly the same point in the pregnancy — the difference is just which reference point you're counting from, not a different due date overall.
The last-period method assumes a standard 28-day cycle with ovulation on day 14. If your actual cycle is longer or shorter, ovulation — and therefore conception — happens on a different day relative to your period, which shifts the true due date slightly. The conception-date method sidesteps this by counting directly from when conception is believed to have occurred, which can be more accurate for people who track ovulation closely or know their exact conception date. The IVF transfer method is generally considered the most precise, since IVF pregnancies have a known, exact embryo age at the time of transfer, removing the guesswork present in the other two methods.
A "due date" represents the estimated midpoint of what's actually a normal range of delivery timing — babies are considered full-term anywhere from 37 to 42 weeks, and only a small percentage of births happen precisely on the calculated due date. Genetics, whether it's a first pregnancy, and various individual health factors all influence exactly when labor begins naturally. This is why healthcare providers use the due date as a planning reference point for scheduling checkups and monitoring growth, rather than as a firm prediction of the exact delivery day.
Pregnancy is typically discussed in weeks rather than months because it allows more precise tracking of fetal development milestones. The first trimester spans weeks 1 through 12, the second trimester spans weeks 13 through 26, and the third trimester spans weeks 27 through birth. Because pregnancy dating starts from the last menstrual period rather than conception, the first two weeks of a pregnancy technically occur before conception has actually happened — an quirk of the dating convention that sometimes confuses people new to pregnancy tracking, but it's the standard method used by healthcare providers worldwide.
As the estimated due date approaches, healthcare providers typically monitor pregnancy more closely, since the range of normal, healthy delivery timing spans several weeks around this point. Babies born between 37 and 38 weeks are generally categorized as "early term," 39 to 40 weeks as "full term," 41 weeks as "late term," and 42 weeks or beyond as "post-term," each carrying slightly different monitoring considerations from a care provider's perspective. This wide range of normal timing is exactly why the due date functions best as a planning and monitoring reference point rather than a specific day to expect delivery.
Which method is most accurate — LMP, conception date, or IVF transfer? IVF transfer dating is generally the most precise since the embryo age is exactly known. Conception-date dating is next most accurate if the date is well established. LMP dating is the most commonly used since it doesn't require knowing the exact conception date, but it's the most sensitive to cycle length variation.
Can an ultrasound change my due date? Yes — early ultrasounds (particularly in the first trimester) are often considered more accurate than LMP-based dating and may lead your provider to adjust your due date based on fetal measurements.
Why does my due date estimate differ slightly between two different calculators? Small differences can come from whether a calculator assumes exactly 280 days from LMP or adjusts slightly for a non-28-day cycle — always use your healthcare provider's confirmed due date as the authoritative figure.
Does this calculator account for irregular cycles? The last-period method assumes a standard cycle length. If your cycles are notably irregular, the conception-date method (if known) will generally give a more reliable estimate.
Can this calculator handle twins or multiples pregnancies? The dating math is the same, though multiples pregnancies are often delivered somewhat earlier than the standard due date on average — your provider will guide monitoring specific to your situation.
How early can this calculator estimate a due date after conception? As soon as you know your last period date, conception date, or IVF transfer date, you can get an estimate — even before a pregnancy is confirmed by testing in some cases.
Last reviewed by Mehmed on July 11, 2026.