A pregnancy is 40 weeks long, counting from the first day of your last menstrual period. That works out to about 280 days, or roughly 9 calendar months. But 40 weeks is a midpoint estimate, not a precise expiration date. Most babies arrive somewhere between 37 and 42 weeks.
Why the Count Starts Before Conception
The 40-week clock doesn’t start when the egg is fertilized. It starts about two weeks earlier, on the first day of your last period. Doctors use this date because most people can recall it reliably, while the exact day of ovulation and conception is harder to pin down. This method assumes a 28-day menstrual cycle with ovulation on day 14, which means the actual age of the baby is roughly two weeks less than the “gestational age” your provider uses. A baby at 40 weeks gestational age has really been developing for about 38 weeks.
If your cycles are irregular, longer, or shorter than 28 days, a dating ultrasound in the first trimester gives a more accurate estimate than the calendar method alone.
How Your Due Date Is Calculated
The standard formula, known as Naegele’s Rule, takes three steps: start with the first day of your last menstrual period, count back three calendar months, then add one year and seven days. So if your last period started on March 10, you’d count back to December 10, then add a year and seven days, giving you a due date of December 17.
Most pregnancy apps and online calculators do this math automatically. Your provider may adjust the date if an early ultrasound suggests the baby is measuring ahead of or behind that estimate.
The Three Trimesters
Pregnancy is divided into three trimesters, each with a distinct phase of development:
- First trimester (weeks 1 through 13): Fertilization happens, and all major organs begin forming. This is when nausea, fatigue, and breast tenderness are most common.
- Second trimester (weeks 14 through 27): A period of rapid growth. The baby starts to move noticeably, and many people feel their best physically during this stretch.
- Third trimester (weeks 28 through 40): The baby gains weight and the organs mature in preparation for life outside the womb. Expect more pressure on your bladder, back discomfort, and increasing fatigue as the due date approaches.
What “Full Term” Actually Means
Not all weeks near the end of pregnancy are equal. The American College of Obstetricians and Gynecologists breaks it down like this:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Postterm: 42 weeks and beyond
These distinctions matter because babies born at 39 weeks and later tend to have fewer breathing problems, better temperature regulation, and stronger feeding ability than those born even a week or two earlier. That’s a big reason providers generally avoid scheduling elective deliveries before 39 weeks.
Preterm Birth Categories
Any birth before 37 completed weeks is considered preterm. The World Health Organization breaks preterm births into three categories based on how early the baby arrives: extremely preterm (before 28 weeks), very preterm (28 to 31 weeks), and moderate to late preterm (32 to 36 weeks). The earlier a baby is born, the more medical support they typically need. Babies born in the late preterm range often do well but may still spend extra time in the hospital for monitoring.
Few Babies Arrive on Their Due Date
The 40-week mark is a best guess, not a deadline. In reality, there’s a wide window of normal delivery times. About half of first-time mothers give birth by 40 weeks and 5 days, meaning the other half go even longer. For people who have given birth before, the midpoint is slightly earlier, around 40 weeks and 3 days.
A study tracking 1,514 healthy women found that 75% of first-time mothers delivered by 41 weeks and 2 days, while 75% of those with a prior birth delivered by 41 weeks. A smaller study of 113 women showed that 10% gave birth by 38 weeks and 5 days, while 90% had delivered by 44 weeks. That five-week spread between the earliest and latest births shows just how variable the timeline is.
Because induction is common at or before 40 weeks in Western countries, it’s hard to know exactly what percentage of pregnancies would naturally go past the due date without intervention. The takeaway: planning around a “due window” of 39 to 41 weeks is more realistic than circling a single date on the calendar.
Why Pregnancy Length Varies
Several factors influence when labor begins naturally. Cycle length plays a role, since the standard 40-week estimate assumes a textbook 28-day cycle. If you typically ovulate later than day 14, your true due date may be later than the formula suggests. Whether this is your first or second pregnancy also matters, as first pregnancies tend to run a couple of days longer on average. Individual biological variation accounts for the rest. Even among women with precisely dated conceptions, natural delivery times can differ by as much as five weeks.