Most babies arrive within a week or two of their due date, but roughly 5 to 15 percent of pregnancies extend beyond 42 weeks. The reasons range from genetics and hormones to something as simple as an inaccurate due date. In many cases, the signals that normally trigger labor just take longer to build up, and no single cause can be pinpointed.
How Labor Normally Starts
Labor isn’t triggered by a single switch. It’s the result of a chain reaction between the baby and the mother’s body that builds over several weeks. The process begins when the baby’s brain activates its own stress-response system, which causes the baby’s adrenal glands to release cortisol. That cortisol does something crucial: it stimulates the baby’s lungs to produce a protein called surfactant protein A, which enters the amniotic fluid.
Once in the fluid, this protein activates immune cells that migrate to the uterus and ramp up inflammation. At the same time, the placenta dramatically increases its production of a stress hormone that boosts prostaglandins, compounds that soften the cervix and make the uterus contract. Meanwhile, progesterone, the hormone that has been keeping the uterus calm for nine months, gradually loses its grip. The combination of rising inflammation and falling progesterone protection is what finally pushes the body into active labor.
When any part of this cascade is slower or weaker than usual, the pregnancy continues past the due date.
The Due Date Itself May Be Wrong
One of the most common reasons a baby appears “late” is that the due date was never quite right. Due dates calculated from the first day of your last menstrual period assume a textbook 28-day cycle with ovulation on day 14. In reality, cycle lengths vary widely. About 28 percent of pregnancies show a difference of more than five days between menstrual-based dating and ultrasound-based dating. Irregular cycles, recent contraceptive use, or early pregnancy bleeding mistaken for a period can all shift the estimated date.
A first-trimester ultrasound is the most reliable way to date a pregnancy, but even that has a margin of error of several days. So some babies labeled “late” were simply conceived a few days later than assumed.
Genetics Play a Measurable Role
The tendency to carry a pregnancy longer runs in families, and it comes from both parents. A large study tracking parents and their children found that for every additional week a mother was overdue at her own birth, her baby’s gestation increased by about 1.2 days. The father’s birth timing mattered too: each extra week in the father’s gestational age at birth added roughly 0.6 days to his child’s gestation.
This makes biological sense. The baby inherits genes from both parents that influence how quickly or slowly that hormonal cascade leading to labor unfolds. And the mother carries her own set of genes, inherited from her mother, that affect how her uterus responds to those signals. If you were born late and your partner was born late, your baby has a higher chance of being late as well.
Maternal Weight and Hormones
Higher body weight during pregnancy is consistently linked to longer gestation. Research comparing thousands of pregnancies found that women with a higher BMI in the first trimester delivered, on average, several days later than leaner women. The effect increased with greater weight gain during pregnancy: women who gained the most weight had an average gestation about 2.5 days longer than those who gained the least.
The likely explanation involves cortisol. In non-pregnant women, there’s a clear inverse relationship between body weight and circulating cortisol levels. During pregnancy, obese women tend to have lower cortisol, which may slow the placenta’s production of the stress hormone that kicks off the labor cascade. Women who delivered past their due date also showed a less rapid rise in this placental hormone during the second trimester compared to women who delivered on time. In other words, the hormonal clock that counts down to labor ticks more slowly.
First Pregnancies and Other Risk Factors
First-time mothers are more likely to deliver late than women who have given birth before. The body has never gone through labor, so the cervix and uterus may take longer to respond to the hormonal signals. Other well-established risk factors include carrying a male baby, which is associated with slightly longer gestation, and having had a previous post-term pregnancy. If your first baby came late, the odds of your second baby coming late are higher than average.
What Happens to the Placenta
The placenta is designed to function for about 40 weeks. Beyond that point, it begins showing signs of aging: reduced cell growth, accumulated cellular debris, and changes in blood flow. These changes don’t happen on a fixed schedule, and many placentas continue functioning well past 41 weeks. But the risk of the placenta no longer keeping up with the baby’s needs does increase with each passing day, which is why healthcare providers monitor late-term pregnancies more closely.
After 41 weeks, monitoring typically involves checking the baby’s heart rate patterns, movement, muscle tone, breathing motions, and the amount of amniotic fluid surrounding the baby. A drop in fluid levels is one of the earliest signs that the placenta is struggling, and it’s one of the main reasons providers recommend delivery rather than continued waiting.
Late-Term Versus Post-Term
Not every overdue pregnancy carries the same level of concern. The American College of Obstetricians and Gynecologists draws a line between “late-term” (41 weeks through 41 weeks and 6 days) and “post-term” (42 weeks and beyond). Most overdue babies fall into the late-term category and do perfectly well. The risks that doctors worry about, including larger birth weight making delivery harder and declining placental function, increase more steeply once a pregnancy crosses 42 weeks.
In practice, fewer and fewer pregnancies actually reach 42 weeks in countries with routine prenatal care, because induction is commonly offered between 41 and 42 weeks. This is a large part of why the reported rate of truly post-term pregnancies varies so much across different healthcare settings. In places where induction is offered earlier, the rate drops significantly.
Why It’s Often Unexplained
Even after accounting for dating errors, genetics, weight, and birth order, many late pregnancies have no identifiable cause. The hormonal cascade that triggers labor involves dozens of interacting signals between the baby, the placenta, and the mother’s immune system. Small variations in any of these can shift the timeline by days or weeks without anything being medically wrong. A due date is a statistical midpoint, not a deadline. About half of all babies arrive after it, and most of them are simply following their own biological clock.