Original timeline
The 40-week gestational calendar
Gestational age convention starts about two weeks before conception in a textbook 28-day cycle.
Three starting dates, three arithmetic rules
| Information available | Calculator arithmetic | Core assumption |
|---|---|---|
| First day of last menstrual period (LMP) | Add 280 days, then adjust for entered cycle length | LMP is reliable and cycle pattern is informative |
| Estimated conception date | Add 266 days | The conception date is reasonably known |
| IVF embryo transfer date | Add 266 days minus embryo age at transfer | Transfer date and embryo age are known |
The pregnancy due-date calculator shows the formula used, estimated LMP-equivalent date, conception estimate, trimester start dates, and an interactive 40-week timeline.
LMP dating and the 280-day convention
Clinical gestational age traditionally counts from the first day of the last menstrual period, not from conception. A 280-day or 40-week count assumes a textbook 28-day cycle with ovulation around day 14. That convention explains why someone can be described as about four weeks pregnant roughly two weeks after conception.
Worked LMP example
First day of LMP: January 1, 2026. Cycle length: 28 days.
January 1 + 280 days = October 8, 2026.
That exact case is included in the calculator’s automated date-arithmetic test.
Irregular cycles and cycle-length adjustments
An LMP-based estimate can be less representative when cycles vary substantially, the date is uncertain, bleeding was not a normal period, hormonal contraception recently changed, or ovulation occurred earlier or later than the textbook assumption.
WellnessCalcs exposes a cycle-length adjustment so the arithmetic is transparent:
For a consistent 32-day cycle, the displayed estimate moves four days later. For a 24-day cycle, it moves four days earlier. This adjustment is a planning convention, not a substitute for clinical dating. If cycles are highly variable, one average number can create false precision.
LMP versus conception dating
When conception is reasonably known, adding 266 days reaches the same theoretical due date as adding 280 days to an LMP exactly 14 days earlier. The two methods differ by the convention’s two-week lead-in.
Sex date and conception date are not necessarily the same. Sperm can remain viable for multiple days, and the precise moment of fertilization is usually not observed. A date selected as “conception” may therefore be less certain than it appears.
IVF transfer dating
IVF provides more precise timing information because the transfer date and embryo age are documented. The calculator subtracts embryo age from the 266-day conception interval:
Day-3 embryo: transfer date + 263 days
Worked day-5 transfer example
Transfer date: January 1, 2026. January 1 + 261 days = September 19, 2026.
Do not replace a fertility clinic’s established date with an online calculation. The example exists to make the code reproducible.
Trimester boundaries on the gestational calendar
WellnessCalcs uses week 14 as the start of the second trimester and week 28 as the start of the third. The dynamic result derives those dates from the estimated LMP-equivalent date:
| Stage | Weeks used by this site | Timeline calculation |
|---|---|---|
| First trimester | 0 weeks through 13 weeks, 6 days | Starts at LMP-equivalent date |
| Second trimester | 14 weeks through 27 weeks, 6 days | LMP-equivalent + 98 days |
| Third trimester | 28 weeks through the due-date estimate | LMP-equivalent + 196 days |
Different resources sometimes summarize months and weeks differently because calendar months are not exactly four weeks. Weeks and days are the clearer clinical convention.
Why a clinical due date can differ
ACOG guidance explains that LMP information and the first accurate ultrasound are used to establish an estimated due date, with decisions depending on gestational age and the size of the discrepancy. Once an official due date is selected, later growth differences do not automatically mean the date should keep moving.
An online calculator cannot review ultrasound measurements, cycle history, fertility records, bleeding history, or clinical judgment. Its role is limited to applying date arithmetic to the date a visitor enters.
What this calculation can and cannot do
It can
- Add the correct interval for LMP, conception, or IVF transfer
- Show the exact formula used
- Map estimated trimester start dates
- Provide reproducible reference cases
It cannot
- Confirm pregnancy or viability
- Interpret ultrasound or symptoms
- Determine when labor will begin
- Replace prenatal care or an official due date
Use appropriate care for symptoms
Do not use a date calculator to evaluate pain, bleeding, fainting, severe illness, or any pregnancy concern. Contact an appropriate healthcare professional or emergency service.
Frequently asked questions
Why does pregnancy dating start before conception?
Gestational age traditionally starts on the first day of LMP. In a textbook cycle, that is about two weeks before conception.
How accurate is an online due-date calculator?
It can apply the arithmetic correctly, but the result depends on whether the entered starting date accurately represents the pregnancy. Clinical dating can differ.
What if my cycles are irregular?
An average cycle adjustment may still create false precision when cycles vary. Discuss dating with prenatal care rather than relying on LMP alone.
Is conception date the same as the date of sex?
Not necessarily. Fertilization can occur after the sex date, and the exact timing is often unknown.
Why is IVF calculated differently?
The embryo already has a known age on transfer day, so that age is subtracted from the 266-day interval.
Does the estimated due date predict the delivery day?
No. It is a reference date for pregnancy dating, not a guarantee of when labor or delivery will occur.
Sources
References and further reading
- American College of Obstetricians and Gynecologists — Methods for Estimating the Due Date
- American College of Obstetricians and Gynecologists — How Your Fetus Grows During Pregnancy
- Office on Women’s Health — Stages of Pregnancy
Method and sources reviewed: July 23, 2026