A full-term pregnancy lasts 40 weeks, counted from the first day of your last menstrual period (LMP). That works out to about 280 days, or roughly 9 calendar months. But those 40 weeks include about two weeks before conception actually happens, which is one of the most confusing things about pregnancy math.
Why Pregnancy Is 40 Weeks, Not 38
Pregnancy is dated from the first day of your last period, not from the day you conceived. In a typical 28-day cycle, ovulation and conception happen around day 14. That means when your doctor says you’re “4 weeks pregnant,” the embryo is really only about 2 weeks old. The actual time a baby develops is closer to 38 weeks, but the 40-week system remains the standard because the start of a period is a date most people can identify, while the exact day of conception is usually unknown.
This dating method is called gestational age, and it’s the number your provider uses for every milestone, test, and appointment throughout pregnancy. If you’ve seen the term “fetal age,” that’s the alternative count starting from conception, which runs about two weeks behind.
How Your Due Date Is Calculated
The most common formula takes the first day of your last period, counts back three calendar months, then adds 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 to get a due date of December 17. This shortcut assumes a regular 28-day cycle with ovulation on day 14, which doesn’t apply to everyone. Irregular cycles, inaccurate recall of your last period, or cycles that are naturally shorter or longer can all shift the real timeline.
An early ultrasound, typically done in the first trimester, is often more accurate than period-based dating. It measures the embryo’s size to estimate gestational age, and providers may adjust your due date based on that measurement.
The Three Trimesters, Week by Week
The 40 weeks are divided into three trimesters, each with a distinct phase of development.
- First trimester (week 1 through week 13): Fertilization occurs, and all major organs begin forming. This is when nausea, fatigue, and breast tenderness are most common. Most miscarriages happen during this period.
- Second trimester (week 14 through week 27): Rapid growth and development. The baby starts moving noticeably, and many early symptoms ease up. This trimester is often described as the most physically comfortable.
- Third trimester (week 28 through week 40): The baby gains significant weight and organs mature in preparation for life outside the womb. For you, this means increasing pressure on your bladder, back pain, and difficulty sleeping as the baby takes up more space.
Not All “Term” Pregnancies Are the Same
Reaching 37 weeks used to be the simple cutoff for a “term” baby, but the medical definition has been refined into narrower categories because outcomes differ meaningfully across those final weeks:
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Postterm: 42 weeks 0 days and beyond
Babies born at 39 or 40 weeks generally have the best outcomes. Those born at 37 or 38 weeks are usually healthy but have slightly higher rates of breathing difficulties and feeding problems compared to babies who make it to 39 weeks. This is why providers typically avoid elective inductions or scheduled deliveries before 39 weeks unless there’s a medical reason.
What Happens If You Go Past 40 Weeks
Going past your due date is common, especially with a first pregnancy. Most women who deliver after 40 weeks have uncomplicated births and healthy babies. Still, risks gradually increase the longer a pregnancy continues. A pregnancy that reaches 42 weeks is classified as postterm, and potential concerns include decreased amniotic fluid, a larger-than-average baby (which can complicate delivery), and in rare cases, stillbirth. The baby can also pass its first stool before birth, which may cause breathing problems if inhaled.
For the mother, going past the due date raises the likelihood of needing an assisted vaginal delivery or cesarean section, along with a higher chance of infection and heavy bleeding after birth. Because of these risks, induction of labor is typically recommended if pregnancy reaches 41 weeks.
Why Some Pregnancies Run Shorter or Longer
The 40-week figure is an average, and healthy pregnancies naturally vary by a couple of weeks in either direction. Several factors influence where you fall on that range.
Things that tend to shorten pregnancy include smoking (the most extensively studied risk factor for preterm birth), heavy alcohol consumption (seven or more drinks per week), high caffeine intake (three or more caffeinated drinks daily), cannabis or cocaine use, high blood pressure, diabetes, carrying multiples, a previous preterm birth, maternal age under 18 or over 40, a short gap between pregnancies (less than six months), and low or high body mass index. Even environmental exposures like air pollution and extreme temperatures have been linked to earlier delivery.
Factors associated with longer pregnancies include being pregnant for the first time, a history of past postterm pregnancies, carrying a male baby, older maternal age, and obesity. Research has also found that biological factors like the timing of implantation play a role: a longer gap between ovulation and implantation is associated with a longer pregnancy overall.
None of these factors are destiny. They shift the odds slightly, and most pregnancies still land within a few days of the 40-week mark regardless.
Preterm Birth Categories
A baby born before 37 weeks is considered preterm. The World Health Organization breaks this into three categories based on how early the baby arrives:
- Moderate to late preterm: 32 to 37 weeks. These babies often do well but may need extra support with feeding or temperature regulation.
- Very preterm: 28 to less than 32 weeks. These babies typically require time in a neonatal intensive care unit.
- Extremely preterm: less than 28 weeks. Survival rates have improved dramatically, but these babies face the highest risk of complications.
About 1 in 10 births worldwide is preterm, with the vast majority falling in the moderate to late preterm range.