What “weeks pregnant” means
Pregnancy is usually described with gestational age, counted from the first day of the last menstrual period (LMP), not from the day fertilization occurred. That convention means the pregnancy clock generally starts about two weeks before conception in a textbook 28-day cycle. It gives clinicians and patients a consistent starting date when the exact conception time is unknown.
A result such as “12 weeks 4 days” means 12 complete weeks have elapsed since the dating start, plus four additional days. It does not mean the pregnancy is already in its 13th completed week; rather, week 13 is in progress.
Pregnancy week formula
Complete weeks = floor(gestational days ÷ 7)
Remaining days = gestational days mod 7
For example, 88 elapsed days equals 12 complete weeks with four days remaining, because 12 × 7 = 84 and 88 − 84 = 4. Use calendar dates rather than assuming every month contains four weeks; most months are longer than 28 days.
Estimated due-date formula
A traditional shortcut called Naegele’s rule adds one year, subtracts three months and adds seven days. Directly adding 280 calendar days is clearer in software and handles month lengths automatically. Both methods are estimates based on LMP dating assumptions.
Worked example
If the first day of the LMP was 1 June 2026, an LMP-based calculator adds 280 days for the initial due-date estimate. On 9 September 2026, 100 days have elapsed from the LMP date, equal to 14 weeks and 2 days. The calculator should show both the gestational age and the estimated due date so the counting basis is visible.
When the LMP estimate can be off
The 280-day method assumes the reported LMP date is reliable and roughly fits a 28-day cycle with ovulation around day 14. Irregular cycles, uncertain bleeding dates, recent hormonal contraception, breastfeeding and later or earlier ovulation can shift the relationship between LMP and conception.
Adding or subtracting cycle length mechanically is only a rough adjustment; it does not prove an ovulation date. Assisted-reproduction pregnancies may use embryo-transfer or procedure dates instead of the ordinary LMP rule.
Why ultrasound dating matters
The American College of Obstetricians and Gynecologists states that first-trimester ultrasound measurement is the most accurate method to establish or confirm gestational age. A clinician may keep the LMP estimate or revise the estimated due date according to ultrasound findings, date reliability and professional guidelines. Once a best obstetric estimate is established and documented, later calculations should use that clinical date instead of repeatedly changing the due date based on a consumer calculator.
Pregnancy weeks and trimesters
Trimesters divide pregnancy into broad clinical periods. Common clinical notation treats the first trimester as extending through 13 weeks 6 days, the second from 14 weeks through 27 weeks 6 days, and the third from 28 weeks onward. Apps sometimes label boundaries differently, so the exact gestational week and day are more precise than only saying “second trimester.”
Gestational age versus fetal age
Gestational age is not the same as time since conception. In a typical cycle, conception may occur about two weeks after the LMP start, so estimated fetal age is often roughly two weeks less than gestational age. This is only an approximation: ovulation timing varies, and intercourse dates cannot by themselves identify the exact fertilization date.
Clinical sources and limitations
The dating method and cautions here follow ACOG’s Methods for Estimating the Due Date. ACOG also describes a typical singleton gestation as averaging 40 weeks, or 280 days, from the first day of the LMP in its Definition of Term Pregnancy.
An online calculator cannot confirm pregnancy viability, diagnose a problem or replace prenatal care. Seek professional care for medical questions, concerning symptoms or uncertainty about dates.
Common calculation mistakes
- Counting from an estimated conception date but labelling the result gestational age.
- Treating four weeks as exactly one calendar month.
- Entering the last day of bleeding instead of the first day of the LMP.
- Ignoring an established ultrasound or assisted-reproduction due date.
- Assuming the 280-day estimate predicts the actual delivery day.
Frequently asked questions
How many weeks pregnant am I from my last period?
Count elapsed days from the first day of the LMP to the selected date, divide by seven and show complete weeks plus remaining days.
Why do pregnancy weeks start before conception?
LMP dating provides a commonly available reference date even though conception usually happens later.
Is a due date exactly 40 weeks?
Forty weeks from LMP is an estimate used for dating. Actual delivery can occur before or after that date.
Should I use LMP or ultrasound?
Use the best obstetric estimate documented by your prenatal clinician; early ultrasound can confirm or revise LMP dating.