A positive test usually comes with one urgent question: when is the baby due? The answer comes from a simple calendar rule you can apply yourself in under a minute, as long as you know what the result really means and where it can go wrong.
Why the count starts before conception
Pregnancy is not counted from the day the baby was conceived. Doctors and midwives count it from the first day of your last menstrual period (often shortened to LMP). This is called gestational age.
Because ovulation happens on average about 14.6 days after the start of the previous period, gestational age includes roughly two weeks during which you were not actually pregnant yet. That is why someone can be “six weeks pregnant” when the embryo was conceived only about four weeks earlier. The two numbers measure different things, and both are correct.
A full-term pregnancy is counted as 40 weeks, or 280 days, from the first day of the last period, with a normal range of 38 to 42 weeks.
The rule behind every due date calculator
The standard method is called Naegele’s rule. In plain words:
- Take the first day of your last period
- Add 7 days
- Then add 9 calendar months
There is an equivalent shortcut that some people find easier: add one year, subtract three months, then add seven days. Both versions land roughly 280 days after the start of the last period.
A real example, step by step
Say the first day of your last period was August 10, 2026.
- Add 7 days: August 17, 2026
- Add 9 months: May 17, 2027 is your estimated due date
You can double-check it by counting days: 21 days left in August, then September through April, then 17 days of May add up to exactly 280 days.
The same LMP tells you how far along you are today. On October 1, 2026, 52 days have passed since August 10, which works out to 7 weeks and 3 days of gestational age. Counted from conception, the embryo is only about 5 weeks old.
If you would rather skip the calendar math, our gestational age calculator by LMP does the same count from your last period date.
Your due date is a window, not an appointment
This is the part most people miss. Only about 4% of babies are born exactly on their due date. Around 80% of births happen between 37 and 41 weeks of gestation.
Applied to the example above, that window looks like this:
- 37 weeks: April 26, 2027
- 40 weeks (the due date): May 17, 2027
- 41 weeks: May 24, 2027
A birth before 37 weeks is considered preterm, and after 42 weeks it is called postterm. Studies also put the average pregnancy at about 283 days from the last period, a few days longer than the 280 days the rule assumes, and the typical spread around the estimate is 13 to 16 days depending on the dating method.
Common mistakes with due date math
Counting from the day of conception
If you know the date you conceived and add 40 weeks to it, you will land about two weeks too late. The 40-week figure already includes the two weeks before ovulation.
Trusting the LMP date over an early ultrasound
Naegele’s rule assumes a fairly regular cycle. An early first-trimester ultrasound, which compares the size of the embryo with reference measurements, is generally considered more accurate, especially with irregular cycles. If your scan gives a different date, that is normal, and your care provider will tell you which one to use.
Treating 280 days as universal
The median length of pregnancy varies from about 276 to 288 days depending on factors such as whether it is a first pregnancy, the mother’s age and ethnic background. The rule gives a useful anchor, not a personal prediction.
Forgetting the IVF adjustment
After IVF there is no natural last period to count from. Gestational age is instead estimated by adding 14 days to the egg retrieval or co-incubation date, so the usual 40-week logic still lines up.
Bottom line: add 7 days and 9 months to the first day of your last period, then think of the result as the middle of a few-week window rather than a single date circled on the calendar.
