Each trimester of pregnancy lasts roughly 13 to 14 weeks. A full pregnancy spans about 40 weeks from the first day of your last menstrual period, and that total is divided into three trimesters of approximately equal length. The exact week cutoffs vary slightly depending on the source, but the medical standard is well established.
The Three Trimesters, Week by Week
The American College of Obstetricians and Gynecologists (ACOG) defines the trimesters as follows:
- 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. You’ll sometimes see the first trimester listed as ending at 12 weeks instead of 13 weeks and 6 days. This is a rounding difference, not a medical disagreement. The total still adds up to roughly 40 weeks, which is considered a full-term pregnancy.
What Happens in the First Trimester
The first trimester is when the most dramatic development happens at the smallest scale. Fertilization occurs, and within weeks, all major organs begin forming. By week 5, a primitive heart and circulatory system are taking shape. By week 6, the heart starts beating and tiny buds appear that will become arms. Leg buds follow around week 7 and begin to take on a paddle-like shape by week 8.
Despite all this activity, the embryo is still tiny by the end of the first trimester. This is also the period when many people experience the most intense symptoms: nausea, fatigue, and breast tenderness. Those symptoms are driven partly by a sharp rise in hormones, including one called relaxin, which peaks at the end of the first trimester before leveling off.
Your body is also changing in ways you can’t feel. By the end of the first trimester, blood flow to your kidneys increases by about 50%, and your heart is already pumping noticeably more blood than before pregnancy. Genetic screening and early ultrasounds are typically offered during this window.
What Happens in the Second Trimester
The second trimester, weeks 14 through 27, is often called the most comfortable stretch of pregnancy. Nausea usually fades, energy returns, and the baby is growing fast enough to start showing. At the start of this trimester, the baby is about 3.5 inches long and weighs roughly 1.5 ounces. By the end, around week 26, the baby has grown to about 9 inches and nearly 2 pounds.
Two important prenatal tests fall in this trimester. The anatomy scan, a detailed ultrasound that checks the baby’s organs and growth, happens between weeks 18 and 22. The glucose tolerance test, which screens for gestational diabetes, is done between weeks 24 and 28.
Your cardiovascular system continues to ramp up during this period. By week 24, cardiac output (the amount of blood your heart pumps per minute) can increase by up to 45% over your pre-pregnancy baseline. Blood volume rises significantly too, typically around 45% above normal levels. Because plasma volume increases faster than red blood cell production, your blood becomes more diluted, which is why mild anemia is common and usually normal during pregnancy.
What Happens in the Third Trimester
The third trimester covers weeks 28 through 40 and focuses on weight gain and organ maturation. At week 28, the baby’s eyelids can partially open, and the nervous system is developed enough to regulate body temperature. The baby also begins practicing breathing movements, which can sometimes be seen on ultrasound.
By week 36, most babies have turned head-down in preparation for delivery. Around week 37, the baby’s head may begin descending into your pelvis, a process sometimes called “lightening” or “dropping.” This can make breathing easier but puts more pressure on your bladder.
Your resting heart rate reaches its peak in the third trimester, typically 10 to 20 beats per minute higher than your pre-pregnancy rate. That’s a 20% to 25% increase, which is why you may feel winded doing things that used to be easy. A full-term pregnancy is defined as lasting between 39 weeks 0 days and 40 weeks 6 days, though labor can begin earlier or later.
The “Fourth Trimester”
You may also hear about a fourth trimester. This isn’t a stage of pregnancy but rather the first 12 weeks after birth. The term was popularized to highlight that the postpartum period deserves the same level of medical attention as pregnancy itself. ACOG now recommends that healthcare providers treat this window as a distinct phase of care, focused on the physical and emotional recovery of the parent, not just the health of the baby.