A full-term pregnancy lasts about 40 weeks and is divided into three trimesters, each roughly 13 to 14 weeks long. The trimesters aren’t perfectly equal, and different medical sources draw the boundaries slightly differently, which can be confusing when you’re trying to figure out exactly where you are. Here’s how the weeks break down.
The Three Trimesters, Week by Week
The American College of Obstetricians and Gynecologists (ACOG) defines the trimesters as follows:
- First trimester: Day 1 of your last menstrual period through 13 weeks and 6 days (about 14 weeks total)
- Second trimester: 14 weeks 0 days through 27 weeks and 6 days (about 14 weeks)
- Third trimester: 28 weeks 0 days through 40 weeks and 6 days (about 13 weeks)
You may see slightly different cutoffs depending on the source. Cleveland Clinic, for instance, starts the second trimester at week 13 rather than week 14. The difference is only a few days, so don’t worry if your provider’s handout doesn’t match another resource exactly. The ACOG boundaries above are the most widely referenced standard.
Why Pregnancy Weeks Start Before Conception
One thing that trips people up is that week 1 of pregnancy isn’t actually the week you conceived. Gestational age is counted from the first day of your last menstrual period (LMP), which is typically about two weeks before ovulation and fertilization. So when your provider says you’re “4 weeks pregnant,” conception likely happened around two weeks ago.
This convention exists because most people can recall when their last period started, while the exact day of conception is harder to pin down. A standard calculation assumes a 28-day cycle with ovulation on day 14. If your cycles are irregular, an early ultrasound (before 14 weeks) is the most accurate way to confirm how far along you are. For pregnancies resulting from fertility treatments like IVF, the date is calculated from the embryo’s age and the day of transfer.
How Your Due Date Is Calculated
The classic method, called Naegele’s Rule, works in three steps: take the first day of your last period, count back three calendar months, then add one year and seven days. That gives you an estimated due date 280 days (40 weeks) from the start of your LMP. It’s a quick estimate, not a guarantee. Only about 5% of babies arrive on their exact due date.
What Happens in Each Trimester
First Trimester (Weeks 1 Through 13)
This is when all the major organs begin forming. By the end of the first trimester, the heart, brain, lungs, and digestive system have their basic structures in place. It’s the period of highest sensitivity to disruption, which is why nausea, fatigue, and food aversions are so common: your body is doing enormous developmental work even though you may not look pregnant yet.
Second Trimester (Weeks 14 Through 27)
Often called the most comfortable stretch of pregnancy, the second trimester is a period of rapid growth. Many people feel their baby’s first movements (called quickening) somewhere between weeks 16 and 20. The detailed anatomy scan, which checks the baby’s organs and structure, is typically scheduled around week 20. By the end of this trimester, the baby is roughly 14 inches long and weighs about two pounds.
Third Trimester (Weeks 28 Through 40)
The focus shifts to weight gain and organ maturation. Fat builds up under the baby’s skin to help regulate body temperature after birth, and the lungs continue developing the ability to breathe air. By week 28, the nervous system can control body temperature and trigger practice breathing movements visible on ultrasound. Around week 36, most babies turn head-down in preparation for delivery, and the head may start descending into the pelvis by week 37. Skin smooths out as fat accumulates, and limbs start to look chubby.
Full Term Isn’t Just “40 Weeks”
The end of pregnancy is more nuanced than a single due date. ACOG breaks it into specific categories:
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Postterm: 42 weeks and beyond
These distinctions matter because babies born at 39 weeks have better outcomes on average than those born at 37 weeks, even though both fall within what used to be broadly called “term.” The labels help guide decisions about whether to schedule a delivery or continue waiting.
What Happens If Pregnancy Goes Past 42 Weeks
A pregnancy that extends past 42 weeks carries increased risks, including decreased amniotic fluid (which can compress the umbilical cord and restrict oxygen), the baby inhaling its first stool into its lungs, and a higher chance of stillbirth. The baby may also grow larger than expected, making delivery more difficult and increasing the likelihood of a cesarean birth.
Most providers begin extra monitoring around 41 weeks, typically checking the baby’s heart rate patterns and amniotic fluid levels once or twice a week. If the pregnancy reaches 41 weeks without signs of labor, induction is commonly recommended to reduce these risks.