Almost every pregnancy begins with the same question, asked to a doctor, a partner, a friend or a search box: when is the baby due? The answer shapes appointment schedules, maternity leave, due-date apps and a fair amount of personal planning, yet it is rarely a single certain day. The Due Date Calculator estimates your due date and your current progress from whichever piece of information you actually have, whether that is the first day of your last period, a dating scan, a known conception date or an IVF embryo transfer.
How to Calculate Your Due Date
The classic method is Naegele's Rule, and it is wonderfully simple: take the first day of your last menstrual period and add 280 days. That is 40 weeks of gestation, the length obstetric guidelines use as the standard for a full-term pregnancy. The rule works because pregnancy is dated from the last period rather than from conception, which usually occurs about two weeks later. The same arithmetic can be written a different way for people who know their conception date: add 266 days, the length of a pregnancy counted from conception itself.
Every method in the calculator is a variation on these two numbers. The ultrasound method takes the gestational age measured at a scan and works forward from the scan date. The IVF method adds 266 days to a transfer date and subtracts the embryo's age at transfer, because the transfer moment is known precisely. Whichever anchor you enter, the calculator measures the days between that anchor and your reference date, then reports your gestational age and the days remaining.
How Many Weeks Pregnant Am I?
This is the question parents ask most often, and the answer comes straight from the calendar. Divide the number of days since your dating anchor by seven: the whole number is the completed week of pregnancy and the remainder is the extra days. In the default last-period method with a reference date of today, the calculator does this split for you, showing the completed weeks and the total days elapsed. If your period started 59 days ago, for example, you are 8 weeks pregnant with 3 days, and the calculator reports exactly that alongside the days remaining until the estimated due date.
The Last Menstrual Period Method
This is the default and most widely used approach. You enter the first day of your last menstrual period, and the calculator adds 280 days to find the due date, assuming a typical 28-day cycle. Because the method depends on a remembered date and a regular cycle, it is most reliable for women who track their periods and least reliable when cycles are irregular or the memory is fuzzy. Providers routinely use this date as the starting point, then confirm or adjust it with an early ultrasound.
The Ultrasound Method
An ultrasound estimates the due date by measuring the baby and comparing the size with typical growth curves for every week of pregnancy. You enter the date of the scan and the gestational age it reported, in weeks and extra days. The calculator then works forward from the scan: the due date is the scan date plus 280 days minus the age it measured. Ultrasound dating is at its most accurate in the first trimester, before individual growth patterns diverge, which is why early scans are considered the most reliable dating tool available.
The Conception Date Method
If you know the date you conceived, the calculator uses the 266-day rule, adding 266 days to that date to reach the due date. The method mirrors the last-period approach but from the other side of the equation: conception usually lands around two weeks after the start of the last period, so the two methods produce dates that are about two weeks apart unless the anchors line up exactly. Gestational age is still counted from the last period for clinical purposes, and the calculator keeps both perspectives consistent.
The IVF Transfer Date Method
In vitro fertilization gives the most precise date of all, because the transfer day is recorded exactly. The calculator adds 266 days to the transfer date and subtracts the age of the embryo on the day it was placed, which you select as day 3, day 5 or day 6. This aligns the transfer with the moment conception would have occurred naturally, so the resulting estimate does not depend on remembering a period or estimating ovulation. For couples going through fertility treatment, this number is usually the one their clinic confirms as the official due date.
Interpreting the Results
The results begin with your gestational age in days, the number that everything else flows from, and the completed weeks of pregnancy, the figure your provider quotes at every appointment. The days and weeks until the due date show how far you have left to travel, and both are clamped to zero once the date has passed rather than drifting into confusing negative numbers. The trimester output places you in the first, second or third stage using the standard boundaries, and the progress gauge fills as the pregnancy moves toward delivery, with the bands coloured by trimester.
Why the Due Date Is Not a Promise
Only about four percent of births actually occur on the estimated due date, which is why clinicians prefer to talk about a normal window. Babies born between 37 and 42 completed weeks are considered full-term in practical terms: those from 37 to 39 weeks are early term, those from 39 to 41 are full term, and those from 41 to 42 are late term. Babies before 37 weeks are preterm and after 42 weeks postterm, and both edges come with extra attention. The due date is a target for planning, not a prediction, and the range around it is the normal experience of nearly every pregnancy.
Cycle Length and Adjusted Estimates
Naegele's Rule assumes a 28-day cycle with ovulation on day 14, but real cycles vary from about 21 to 45 days. A longer cycle pushes ovulation, and therefore conception, later, so the due date should shift forward by roughly the difference between your cycle length and 28 days. A shorter cycle shifts it the other way. If your cycles are longer than four weeks, asking for an adjusted estimate based on your own average cycle length gives a more realistic date than the flat 280-day rule, and the earliest ultrasound will generally settle the question regardless.
Weeks Pregnant versus Months Pregnant
Medical professionals work in weeks, but friends and family tend to ask about months, and the two systems never line up into tidy whole numbers. A month is not four weeks, and pregnancy month boundaries depend on the starting date, so the same pregnancy can be called six months, seven months or something in between depending on who is counting. The weeks figure is the reliable currency: it is how the reference date, the dating anchor and the estimated due date all connect, and it is the number your provider compares against growth milestones. If someone asks how many months you are, take your completed weeks and round to the nearest four-week block for a casual answer, but quote the exact weeks whenever precision matters.
A Handy Set of Milestones
Along the way, a handful of routine checks mark the pregnancy's progress, and knowing roughly when they come makes every appointment less mysterious. The dating scan, usually in the first trimester, establishes or confirms the gestational age. The nuchal translucency screening happens around 11 to 14 weeks, and the anatomy scan is commonly scheduled between 18 and 22 weeks to examine the baby's development in detail. A glucose screening test typically lands between 24 and 28 weeks, and growth scans may follow in the third trimester if the provider wants a closer look. Feed your dating anchor into the calculator and set the reference date to any upcoming appointment to see how many weeks pregnant you will be when you walk through the door.
When the Dates Do Not Agree
It is common for the last-period estimate and an ultrasound measurement to differ by several days, and when they do, providers usually trust the scan. First-trimester measurements are built on growth data that is highly consistent this early, so a scan done before about 13 weeks carries the most weight, while a calendar-based estimate carries the uncertainty of a remembered date and an assumed cycle length. If the disagreement is more than about a week, the due date is typically adjusted to the ultrasound estimate, and from then on the whole pregnancy calendar is re-anchored. If the two agree within a few days, the provider simply confirms the date. Either way, the calculator's ultrasound method lets you re-derive every remaining output, weeks pregnant and days to go, from the scan date alone once the clinic settles on a number.
Short Connections Worth Making
The tools around this one behave as a family. The ovulation calculator and the period calculator work forward from a cycle to predict fertile days, the conception calculator finds a likely conception window from a due date in the other direction, and the pregnancy calculator tracks milestone progress through the weeks. The weight gain calculator converts a starting weight, a height and current weeks into a recommended gain range for the rest of the pregnancy. Used together they cover most of the planning a pregnancy inspires, from figuring out when you conceived to knowing what your weight target should be at any given week.
Using the Calculator with Your Provider
The calculator is a planning tool, and its result should be confirmed with the people who actually measure your pregnancy. Your provider will check the date against an ultrasound, consider your cycle history and adjust when the two disagree. This matters most when periods are irregular, when dates are uncertain, or when a pregnancy is high-risk and the exact week determines critical decisions about testing, medication and delivery timing. Bring your due date and your weeks pregnant figure to the appointment, and let the scan do the fine-tuning.
Common Questions and Reassurances
- A two-week gap between last-period and conception methods is normal and expected, not an error.
- Going past the due date by a week or two is common and usually harmless, though providers monitor after 41 weeks.
- An early ultrasound can move a calendar-based date by several days or more, and that is a correction, not a contradiction.
- Freaking out at 9 weeks and 6 days when an app says 10 weeks is misplaced: completed weeks are what officially count.
- The reference date defaults to today but can be changed to any date, so you can look ahead to a future appointment.
Whether your pregnancy is dated from a remembered period, a scan photograph or a fertility clinic's records, the Due Date Calculator turns whatever you know into the two numbers everyone wants, your current week and your remaining days, and plants your family's countdown on a solid calendar anchor.
Disclaimer
Results are provided as estimates for informational purposes only and may be inaccurate. Always verify outcomes with a qualified professional before making financial or personal decisions based on these calculations.