A standard pregnancy lasts 280 days, or 40 weeks. That number is counted from the first day of your last menstrual period, not from the day you actually conceived. Since conception typically happens about two weeks into your cycle, the baby is really developing for roughly 266 days, but the 280-day figure is what your provider uses to set your due date.
Why the Count Starts Before Conception
It seems strange that your pregnancy clock starts ticking before you’re actually pregnant, but there’s a practical reason. Pinpointing the exact day of conception is nearly impossible for most people. Ovulation timing varies, sperm can survive for days, and unless you’re tracking very precisely, you won’t know which day the egg was fertilized. The first day of your last period, on the other hand, is something most people can recall.
This is why a positive pregnancy test at two weeks after ovulation already puts you at “four weeks pregnant.” Those first two weeks of the 280-day timeline are the time between your period starting and ovulation occurring. It’s a quirk of the system that confuses almost everyone, but it’s consistent enough to work as a medical standard.
How Accurate Is Your Due Date?
Only about 5% of babies arrive on their exact due date. Most are born somewhere between 37 and 41 weeks, typically within a week of the predicted date. The 280-day estimate assumes a textbook 28-day menstrual cycle with ovulation on day 14, and many people don’t fit that pattern.
A first-trimester ultrasound can sharpen the estimate. Measuring the embryo’s length between 7 and 13 weeks is accurate to within 5 to 7 days. In one study, 40% of women who received a first-trimester ultrasound had their due date adjusted by more than 5 days compared to period-based dating alone. If your cycles are irregular or you’re unsure about your last period, an early ultrasound gives a more reliable timeline.
What Counts as “Full Term”
Not all pregnancies that reach the general neighborhood of 40 weeks are considered the same. Current medical guidelines break the final stretch into distinct categories:
- Early term: 37 weeks through 38 weeks and 6 days (259 to 272 days)
- Full term: 39 weeks through 40 weeks and 6 days (273 to 286 days)
- Late term: 41 weeks through 41 weeks and 6 days (287 to 293 days)
- Post-term: 42 weeks and beyond (294+ days)
These categories matter because babies born in the full-term window (39 to 40 weeks) generally have the best outcomes. Babies born at 37 or 38 weeks are usually healthy, but they have slightly higher rates of breathing difficulties and feeding challenges compared to those who make it to 39 weeks. That’s why providers typically won’t schedule an elective delivery before 39 weeks unless there’s a medical reason.
What Happens if You Go Past 280 Days
Going a few days or even a week past your due date is common and usually not dangerous. But the longer a pregnancy continues beyond 41 weeks, the more closely your provider will monitor you. Post-term pregnancies (42 weeks and beyond) carry a small but real increase in risks: the placenta becomes less efficient at delivering nutrients and oxygen, amniotic fluid levels can drop, and the baby may grow larger than expected, making delivery more difficult.
Specific concerns with post-term pregnancy include the umbilical cord becoming compressed as fluid decreases, the baby inhaling its first stool (which can cause serious breathing problems after birth), and a higher likelihood of needing a cesarean or assisted vaginal delivery. For the mother, post-term pregnancy raises the chance of infection and heavy bleeding after birth. Problems occur in only a small number of post-term pregnancies, but most providers will discuss induction once you reach 41 weeks.
The Short Answer
Pregnancy is 280 days (40 weeks) counted from your last menstrual period, or about 266 days from conception. Your actual delivery will most likely fall somewhere between 259 and 293 days, with the ideal window sitting right around that 273-to-286-day mark. The due date is a best estimate, not a deadline.