A standard pregnancy lasts 40 weeks, or 280 days. 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 growing for roughly 38 weeks, but the official count is always 40.
Why the Count Starts Before Conception
Pregnancy math can feel counterintuitive. During the first two weeks of your “pregnancy,” you aren’t actually pregnant yet. Doctors use the first day of your last period as the starting point because most people can remember that date, while pinpointing the exact day of conception is much harder. This counting method, called gestational age, is the standard worldwide.
The actual age of the developing baby, sometimes called fetal age, runs about two weeks behind gestational age the entire pregnancy. So when your provider says you’re 12 weeks pregnant, the embryo has been developing for closer to 10 weeks.
How Your Due Date Is Calculated
The most common formula, known as Naegele’s Rule, works like this: take the first day of your last period, count back three calendar months, then add one year and seven days. That gives you a date roughly 40 weeks out. The formula assumes a 28-day menstrual cycle, so if your cycles are longer or shorter, the estimate shifts accordingly.
Early ultrasounds can also estimate your due date by measuring the embryo’s size. When there’s a significant gap between the ultrasound estimate and the one based on your period, your provider will typically adjust. Either way, a due date is an estimate, not an appointment. The window for normal delivery spans several weeks on either side.
The Three Trimesters
Pregnancy is divided into three roughly equal trimesters, each with distinct developments:
- First trimester (weeks 4 through 12): All major organs begin forming. This is when nausea, fatigue, and breast tenderness are most common. The risk of miscarriage is highest during this stretch.
- Second trimester (weeks 13 through 27): Often called the most comfortable trimester. The baby grows rapidly, you start to show, and most people feel movement for the first time around weeks 18 to 22.
- Third trimester (weeks 28 through 40 or 41): The baby gains significant weight, the lungs mature, and your body prepares for labor. Discomforts like back pain, swelling, and difficulty sleeping tend to increase.
What “Full Term” Actually Means
Not all weeks are treated equally when it comes to delivery timing. The American College of Obstetricians and Gynecologists breaks it down into specific categories:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Post-term: 42 weeks and beyond
These distinctions matter because babies born even a couple of weeks early can face more breathing difficulties and feeding challenges than those born at 39 weeks or later. A pregnancy that reaches 39 weeks is considered the sweet spot for healthy outcomes.
Why Some Pregnancies Run Longer or Shorter
Plenty of variation exists within the 40-week framework. Several factors are linked to longer pregnancies: being pregnant for the first time, older maternal age, carrying a male baby, higher body weight, and having had a previous pregnancy that went past the due date. If your first pregnancy went to 41 or 42 weeks, your next one has a higher chance of doing the same.
Interestingly, research that measured pregnancy length from the precise day of ovulation (rather than the last period) found that the time from ovulation to implantation also plays a role. Embryos that took longer to implant in the uterine wall were associated with longer overall pregnancies. This suggests some of the variation is baked in from the very earliest days.
What Happens if You Pass 40 Weeks
Going past your due date is common, especially with a first baby. Your provider will typically increase monitoring after 40 weeks, checking the baby’s heart rate and fluid levels to make sure everything remains healthy. If your pregnancy reaches 41 weeks, labor induction is generally recommended. The goal is to balance giving the baby time to develop naturally with avoiding the increased risks that come with post-term pregnancy, including a larger baby, lower amniotic fluid, and reduced placental function.
Most babies arrive somewhere between 37 and 42 weeks. The 40-week mark is the midpoint of a range, not a deadline. Your body, your baby’s development, and your individual health picture all influence when labor begins.