Forty weeks is the standard length of a full-term pregnancy, counted from the first day of your last menstrual period. That works out to 280 days, or roughly nine calendar months. If you’re at 40 weeks right now, you’re at your estimated due date, but only about half of women actually deliver by this point.
How the 40-Week Timeline Is Calculated
The 40-week count doesn’t start at conception. It starts on the first day of your last period, which is typically about two weeks before you actually ovulated and conceived. This means the baby has really been developing for closer to 38 weeks, but the medical world uses the 40-week convention because the date of your last period is easier to pinpoint than the date of conception.
The standard method for calculating a due date is called Naegele’s Rule: take the first day of your last period, count back three calendar months, then add one year and seven days. This assumes a 28-day menstrual cycle, so if your cycles are longer or shorter, the estimate shifts accordingly. Early ultrasounds can also adjust the date based on the baby’s size.
How Many People Actually Deliver at 40 Weeks
Very few babies arrive exactly on their due date. In a well-known 2001 study of healthy women, 50% of first-time mothers gave birth by 40 weeks and 5 days, while 75% delivered by 41 weeks and 2 days. Women who had given birth before tended to deliver slightly earlier: half by 40 weeks and 3 days.
A later study using more precise methods found similar results. Half of participants gave birth by 40 weeks and 5 days after their last period, 25% hadn’t delivered until after 41 weeks and 2 days, and 10% were still pregnant at 44 weeks. The takeaway: your due date is a midpoint on a bell curve, not a deadline. Going past it is normal.
What Full Term Actually Means
Doctors don’t treat every week near the due date the same. The official classification breaks it down like this:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Post-term: 42 weeks and beyond
At exactly 40 weeks, you’re squarely in the full-term window. Babies born during this range generally have the best outcomes, which is why elective inductions and scheduled cesareans aren’t recommended before 39 weeks unless there’s a medical reason.
Baby’s Size and Development at 40 Weeks
A baby born at 40 weeks weighs about 7 pounds 4 ounces (3,300 grams) on average and measures around 20 inches (51 cm) long. By this point, all major organs are developed. The lungs are ready to breathe air, the brain has been rapidly building neural connections throughout the third trimester, and the baby has built up enough body fat to regulate temperature outside the womb.
The baby’s position matters too. Most have settled head-down by now, low in the pelvis. You may notice the pressure has shifted downward, sometimes called “lightening,” which can make breathing easier but increase pressure on your bladder.
What Your Body Is Doing at 40 Weeks
Your cervix is preparing for labor through a process called ripening, where it gradually softens, thins out, and begins to open. This can happen over days or weeks before active labor starts. As the cervix changes, you may notice an increase in vaginal discharge that’s clear, pink, or slightly bloody. This is the mucus plug that sealed the cervix during pregnancy, and losing it is one early sign that your body is getting ready.
Hormonal shifts are accelerating. Your body is producing more of the hormones that trigger contractions, while the baby’s own hormonal signals play a role in initiating labor. You may experience Braxton Hicks contractions more frequently, which are irregular tightening sensations that don’t follow a pattern and often stop when you rest or drink water.
Recognizing True Labor
The key difference between practice contractions and real labor is the pattern. True labor contractions come at regular intervals, get closer together over time, and don’t go away when you rest or hydrate. They may feel like intense menstrual cramps in your lower back or pelvis.
Your water breaking is the other major sign. You may feel a gush of fluid or a slow trickle. Not everyone’s water breaks before contractions start; for many women, it happens well into active labor or is broken by a healthcare provider at the hospital. You should head to the hospital if your water breaks without contractions, if you’re bleeding heavily, if the pain is constant with no breaks between contractions, or if you notice the baby is moving less than usual.
What Happens If You Go Past 40 Weeks
Going a few days or even a week past your due date is common and usually not a concern. But the risks do gradually increase the longer pregnancy continues. Post-term pregnancies (42 weeks and beyond) carry higher chances of stillbirth, the baby growing unusually large (which complicates delivery), decreased amniotic fluid, and the baby passing meconium (its first stool) into the amniotic fluid before birth, which can cause serious breathing problems if inhaled.
Lower amniotic fluid is a particular concern because it can allow the umbilical cord to become compressed, restricting oxygen flow to the baby. There’s also a higher chance of needing an assisted vaginal delivery or cesarean, and an increased risk of infection and heavy bleeding after delivery for the mother.
These complications happen in only a small number of post-term pregnancies, but because the risks are real, most providers will begin monitoring more closely. This typically involves checking the baby’s heart rate patterns and measuring amniotic fluid levels, often twice a week once you pass your due date. Research has shown that testing twice weekly is significantly better at catching problems than testing once a week.
Induction After 40 Weeks
If you pass 41 to 42 weeks without going into labor spontaneously, induction is generally recommended. Before that point, induction may be offered for medical reasons: high blood pressure, preeclampsia, gestational diabetes, problems with the placenta, low amniotic fluid, poor fetal growth, or a uterine infection.
Elective induction, where you and your baby are healthy but you choose to be induced, is an option starting at 39 weeks. Some women request it because of physical discomfort, a history of fast deliveries, or living far from the hospital. Research has also shown that elective induction at 39 weeks for healthy women can reduce the chance of cesarean birth, which has shifted how many providers approach the conversation around due dates.
If you’re sitting at exactly 40 weeks with no complications, the most common approach is watchful waiting with regular monitoring, while discussing your preferences for how long to wait before considering induction.