A full-term pregnancy lasts about 40 weeks, divided into three trimesters of roughly 13 to 14 weeks each. The first trimester covers weeks 1 through 13, the second runs from week 14 through week 27, and the third spans week 28 through week 40. Those boundaries aren’t arbitrary. Each trimester represents a distinct phase of fetal development, and knowing where you fall helps you understand what’s happening inside your body right now.
The Standard Week Ranges
The American College of Obstetricians and Gynecologists (ACOG) defines the trimesters with precise cutoffs:
- First trimester: First day of your last menstrual period through 13 weeks and 6 days
- Second trimester: 14 weeks and 0 days through 27 weeks and 6 days
- Third trimester: 28 weeks and 0 days through 40 weeks and 6 days
That makes the first trimester about 14 weeks long, the second trimester exactly 14 weeks, and the third trimester 13 weeks if you deliver right on your due date. You may see some sources end the first trimester at 12 weeks instead of 13 weeks and 6 days, which creates confusion. The ACOG definitions above are the ones your provider is most likely using.
Why Pregnancy Starts Before Conception
One thing that trips people up is that week 1 begins on the first day of your last menstrual period, not the day you conceived. Fertilization typically doesn’t happen until around week 3, roughly 14 days into your cycle. This means you’re already considered “2 weeks pregnant” at the time of ovulation and about “4 weeks pregnant” when you miss your period and get a positive test.
This convention exists because most people can remember when their last period started but can’t pinpoint the exact day of conception. Your due date is calculated as 280 days (40 weeks) from that first day. The system assumes a regular 28-day cycle with ovulation on day 14, so if your cycles are longer or shorter, an early ultrasound can adjust the estimate.
First Trimester: Weeks 1 Through 13
The first trimester is when every major organ system begins to form. After the fertilized egg implants in the uterine wall around week 4, development accelerates quickly. By week 5, three distinct cell layers have formed that will become the skin, nervous system, heart, bones, kidneys, lungs, and intestines. The heart and a basic circulatory system start taking shape. By week 6, the brain and spinal cord are developing from the neural tube along your baby’s back, and tiny arm buds appear.
Things move fast from there. By week 8, fingers begin forming, the nose and upper lip are visible, and your baby is about half an inch long. By week 9, toes appear and eyelids start to form. By the end of week 10, the elbows can bend. Because all the critical organ formation happens during these weeks, the first trimester is the period when a developing baby is most vulnerable to disruptions.
For you, this trimester often brings nausea, fatigue, breast tenderness, and frequent urination as hormone levels surge. The pregnancy hormone HCG rises rapidly starting around week 5, which is what triggers many of those early symptoms.
Second Trimester: Weeks 14 Through 27
The second trimester is the phase of rapid growth and refinement. The skeleton starts to harden, beginning with the skull and the long bones of the arms and legs around week 13. By week 14, your baby’s sex may become visible on ultrasound, and red blood cells are forming in the spleen.
Around weeks 16 through 18, limb movements become coordinated, the ears approach their final position, and your baby may start to hear sounds as the digestive system begins working. Many people first feel fetal movement somewhere around this stretch. By week 20, your baby has a regular sleep-wake cycle and can be woken by noise or your movement.
The later weeks of this trimester bring increasingly detailed development. By week 22, eyebrows and hair are visible, and the reproductive organs are in place. By week 23, fingerprint and footprint ridges are forming, and the lungs begin producing surfactant, a substance they’ll need to breathe air after birth. By week 27, the nervous system is continuing to mature and your baby is gaining fat, filling out beneath skin that had been thin and translucent just weeks earlier.
Third Trimester: Weeks 28 Through 40
The third trimester is primarily about weight gain and organ maturation. At week 28, your baby weighs about 2.25 pounds and is roughly 10 inches long (measured from the top of the head to the rump). The nervous system has developed enough to regulate body temperature and trigger rhythmic breathing movements visible on ultrasound.
Growth accelerates through the 30s. By week 31, most major development is complete and the focus shifts to putting on weight quickly. Your baby roughly doubles in size between week 28 and week 34, reaching about 4.5 pounds. By week 35, there’s much less room to move, and most babies have turned head-down by week 36. If yours hasn’t, your provider may discuss options for repositioning around week 37, which is also when the baby’s head may start descending into your pelvis in preparation for birth.
At week 40, the average baby measures about 14 inches crown to rump and weighs around 7.5 pounds, though healthy weights range widely. Some babies are close to 9 pounds by week 38.
What “Full Term” Actually Means
Your due date falls at 40 weeks, but a baby born anywhere from 37 to 42 weeks is broadly considered term. The distinction is more nuanced than that, though. ACOG breaks it into four 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 at 39 weeks and beyond generally have better outcomes than those born at 37 or 38 weeks. The brain, lungs, and liver undergo important final maturation in those last couple of weeks. If your pregnancy reaches 41 weeks without labor starting on its own, induction is typically recommended to reduce the risks associated with going post-term.