A gestation period is the length of time between conception and birth. In humans, it lasts about 280 days (40 weeks) when counted from the first day of the last menstrual period, or roughly 266 days from the actual moment of fertilization. The term applies to all mammals, and the duration varies enormously across species, from 20 days in mice to nearly 640 days in elephants.
How Human Gestation Is Measured
Human pregnancy is dated from the first day of the last menstrual period (LMP), not from the day of conception. That might sound odd, since a person isn’t actually pregnant during the first two weeks of that count. But because most people can identify when their last period started more reliably than when they ovulated or conceived, this convention has become the standard. It assumes a 28-day menstrual cycle with ovulation on day 14, which means roughly two weeks are “added” to the true biological pregnancy length.
This method has clear limitations. It doesn’t account for irregular cycles, late ovulation, or inaccurate recall of when that last period began. For pregnancies conceived through assisted reproductive technology like IVF, the due date is instead calculated from the known date of embryo transfer and the age of the embryo, which is far more precise.
Ultrasound measurements in the first trimester are now routinely used to confirm or adjust the estimated due date, especially when there’s a discrepancy with the LMP-based calculation.
What Counts as “Full Term”
Not all deliveries at or near 40 weeks are considered the same. Medical organizations now use four distinct categories:
- 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
These distinctions matter because babies born even a week or two before 39 weeks face slightly higher risks of breathing difficulties and feeding problems compared to those born during the full-term window. The old habit of calling anything after 37 weeks “term” has been replaced by this more nuanced framework.
What Makes Gestation Shorter or Longer
Many factors push a pregnancy toward earlier or later delivery. Globally, about 1 in 10 babies are born preterm (before 37 weeks), with rates ranging from 4% to 16% depending on the country. In 2020, an estimated 13.4 million babies were born preterm worldwide.
Tobacco use is one of the most studied risk factors for preterm birth, and secondhand smoke exposure carries risk as well. Drinking seven or more alcoholic drinks per week, consuming three or more caffeinated drinks per day, and using cannabis or cocaine are all associated with modestly increased risk. Beyond substance use, risk factors include a prior preterm delivery, infections during pregnancy, high blood pressure, carrying more than one baby, being under 18 or over 40, having a BMI that is very low or very high, diabetes, uterine fibroids, and getting pregnant again within six months of a previous delivery. Environmental exposures like air pollution, contaminated drinking water, indoor mold, and extreme temperatures have also been linked to preterm birth.
On the other end of the spectrum, pregnancies that extend past 41 or 42 weeks are more common in first-time mothers, those carrying a male fetus, people with a history of previous post-term pregnancies, and those with obesity. When researchers measure pregnancy length from ovulation rather than the LMP, older maternal age and a longer gap between ovulation and implantation are both associated with longer pregnancies.
Viability During Gestation
Viability refers to the point at which a baby could potentially survive outside the uterus with medical support. There is no single fixed threshold, but the periviable period spans weeks 20 through 25. Survival rates rise steeply across this window. Births before 23 weeks carry roughly a 5 to 6% survival rate, and nearly all survivors at that stage face serious long-term health complications. At 23 weeks, survival ranges from 23% to 27%. By 24 weeks, it climbs to 42% to 59%, and at 25 weeks, 67% to 76% of babies survive to hospital discharge.
These numbers reflect what’s possible in hospitals equipped with advanced neonatal intensive care. Outcomes depend heavily on the resources available and on individual circumstances, which is why viability is treated as a clinical judgment rather than a hard cutoff.
How Gestation Varies Across Species
Gestation length in mammals is closely tied to body size and, more specifically, to brain development. Larger animals with bigger brains generally need longer pregnancies. African elephants hold the record at around 640 days, nearly two full years. Sperm whales gestate for about 535 days, and giraffes for 430. Horses come in at 336 days, dolphins at 318.
Humans, at roughly 270 days, fall within the range of other large primates. Chimpanzees gestate for about 240 days. Moving down in size, grizzly bears carry for 215 days, sheep for 148, and lions for 108. Dogs and cats have remarkably similar gestation periods of about 60 days. Rabbits need only 33 days, and mice just 20.
Marsupials like kangaroos and wombats are outliers. A kangaroo’s gestation is only 42 days, and a wombat’s is 27, because their young are born at an extremely early stage of development and continue growing in the pouch. This reflects a fundamentally different reproductive strategy from placental mammals, where the intimate physiological connection between mother and fetus during gestation allows for more energy transfer and, consequently, more brain growth before birth.
Why Brain Size Drives Gestation Length
The relationship between gestation and brain development goes deeper than just “bigger animals take longer.” A mother’s metabolic rate plays a key role. In placental mammals, the close connection between maternal and fetal blood supply allows the mother to funnel metabolic energy directly into growing the fetal brain. A higher maternal metabolic rate can support faster brain growth, but growing a large, complex brain still requires extended time in the womb.
Brain tissue is extraordinarily expensive to maintain. It demands a disproportionate share of the body’s energy. This creates an evolutionary balancing act: species that evolve larger brains must either increase their overall metabolic rate or reduce the energy demands of other organs (like the gut) to compensate. Human brains are about six times larger than you’d expect for a mammal of our size, which helps explain why our gestation is as long as it is, and why brain development continues for years after birth.