Find Due Date Calculator
Enter the first day of your last menstrual period to estimate your due date, conception date, fertility dates, and 12- and 27-week pregnancy milestones.
Pregnancy Due Date and Milestone Estimator
Find Your Due Date Formula
Where:
- \(\text{LMP}\) — the first day of your last menstrual period.
- \(\text{days}\) — calendar days added to the LMP date.
The calculator uses fixed offsets based on a typical 28-day menstrual cycle. It does not adjust its dates for your individual cycle length or the date you actually ovulated.
Find Your Due Date – Calculation Example
Example: If the first day of your last menstrual period was September 28, 2026, add the calculator’s fixed number of days for each milestone.
\[\begin{aligned}
\text{Fertility dates: }&\text{September 28} + 10\text{ to }17\text{ days} = \text{October 8–15, 2026} \\
\text{Conception: }&\text{September 28} + 14\text{ days} = \text{October 12, 2026} \\
\text{12 weeks: }&\text{September 28} + 84\text{ days} = \text{December 21, 2026} \\
\text{27 weeks: }&\text{September 28} + 189\text{ days} = \text{April 5, 2027} \\
\text{40 weeks: }&\text{September 28} + 280\text{ days} = \text{July 5, 2027}
\end{aligned}\]
Result: The estimated due date is July 5, 2027. The other dates are estimates based on the same September 28 LMP.
How to use the calculator
- In First day of your last menstrual period, select the date your most recent menstrual bleeding began—not the date it ended. The field may initially show today’s date, so change it if necessary.
- Read Estimated Fertility Dates as a date range and Estimated Conception Date as a single estimated date.
- Check First Trimester Ends (12 Weeks), Second Trimester Ends (27 Weeks), and Estimated Due Date (40 Weeks) for the remaining milestones.
The dates appear in your device’s local date format. For a more detailed look at pregnancy timing, see the Gestational Age Calculator.
What your estimated due date means
Pregnancy is commonly dated from the first day of the last menstrual period, even though conception usually happens later. On this method, an estimated due date is 40 weeks, or 280 days, after that first day. It is a planning estimate, not a prediction that labor will start on a particular date. Many babies are born before or after their estimated due date.
The conception estimate assumes ovulation around 14 days after the period began. The fertility range runs from day 10 through day 17 after the LMP date. These are broad calendar estimates: sperm can survive for several days, while the timing of ovulation differs between people and can vary from one cycle to another. The displayed conception date cannot establish when conception actually occurred. If you want to explore cycle timing separately, use the Calendar of Ovulation and Fertility.
The 12- and 27-week results mark the exact gestational ages named in their labels. Trimester boundaries are described differently by different sources, so these dates should be read as calculator milestones rather than universal clinical cutoffs. Your prenatal care team may use different wording or revise your pregnancy dating. The How Many Weeks Pregnant Are You? calculator can help you check an LMP-based week count for a particular day.
Assumptions and practical limitations
- Cycle length is fixed in the calculation. There is no cycle-length input. If your cycles are shorter or longer than 28 days, or ovulation happened earlier or later than assumed, the fertility, conception, and due-date estimates may be off.
- The LMP needs to be reliable. If you are unsure when your last period started, have irregular bleeding, or became pregnant soon after stopping hormonal contraception, an LMP-based date may be less useful.
- Clinical dating may differ. A clinician may use an early ultrasound and your medical history to confirm or revise the estimated due date. Follow the date used by your prenatal care team when planning care.
- Fertility dates are not birth-control advice. Do not use this range to decide which days are safe for unprotected sex. Calendar estimates cannot reliably identify or exclude fertile days for an individual cycle.
If you have a positive pregnancy test or questions about your dates, discuss the estimate with a healthcare professional. The calculator provides a starting point for understanding a pregnancy timeline; it cannot confirm pregnancy, determine the exact conception date, or replace prenatal assessment.
Frequently Asked Questions
How can I calculate a due date by hand?
A quick method is Naegele’s rule: take the first day of your last period, subtract three calendar months, add seven days, and add one year. For an exact match with this calculator, count forward 40 weeks on a calendar instead. The two methods can differ slightly because calendar months have different lengths.
Why am I called four weeks pregnant if conception was about two weeks ago?
Pregnancy weeks usually measure gestational age, which starts on the first day of the last menstrual period. The time since fertilization, sometimes called embryonic or fetal age, is typically about two weeks less in a 28-day cycle. That difference is why a pregnancy test may first turn positive when you are already considered around four weeks pregnant.
What if my menstrual cycle is longer or shorter than 28 days?
Ovulation may happen later in a longer cycle or earlier in a shorter one, so an LMP-based due date can shift. A rough adjustment is to move the estimate by the difference between your usual cycle length and 28 days—for example, about seven days later for a 35-day cycle. This is only a rule of thumb; your clinician may date the pregnancy differently.
What if my ultrasound due date differs from my period-based due date?
An early ultrasound can provide a useful dating estimate, particularly if your cycle is irregular or the date of your last period is uncertain. Small differences between methods are common. Your prenatal care team will decide whether to keep or revise the due date based on the scan timing, the size measurements, and your history; avoid switching between dates on your own.
Is it normal to give birth before or after the due date?
Yes. The due date is a reference point, not an appointment. Birth at 39 weeks through 40 weeks and 6 days is generally called full term; 37–38 weeks is early term, and 41 weeks is late term. Your care team can explain what your particular timing means and when they recommend additional monitoring.
How is a due date calculated after IVF?
IVF dating usually uses the embryo transfer date and the embryo’s age rather than the last menstrual period. A common calculation adds 261 days to a five-day embryo transfer or 263 days to a three-day embryo transfer. Confirm the date with your fertility clinic, especially if you have multiple transfer or treatment dates in your records.