Pregnancy is divided into three trimesters, each roughly 13 to 14 weeks long, spanning a total of about 40 weeks. The divisions aren’t arbitrary. They reflect distinct phases of fetal development, from organ formation to rapid growth to final maturation before birth.
The Week-by-Week Breakdown
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
You may notice slight variations from different sources. The UK’s National Health Service, for example, places the first trimester at weeks 4 through 12 and the second trimester starting at week 13. Most major health organizations agree the third trimester starts at week 28, though some round down to 27. These small discrepancies don’t change your care. Your provider will track your pregnancy by exact weeks and days regardless of which trimester label applies.
Why the Count Starts Before Conception
One detail that confuses a lot of people: the pregnancy clock starts on the first day of your last menstrual period (LMP), not the day you conceived. Since ovulation typically happens around day 14 of a 28-day cycle, you’re already considered “2 weeks pregnant” at the point of conception. This means gestational age always runs about two weeks ahead of the actual age of the embryo.
Doctors use this system because most people can pinpoint their last period more reliably than the exact day of ovulation. The result is a standard 280-day (40-week) timeline measured from the LMP, even though the actual time from conception to birth averages closer to 268 days, or about 38 weeks and 2 days.
What Happens in Each Trimester
First Trimester: Organ Formation
The first trimester is when all the major organs form. By the end of this phase, the embryo (now called a fetus starting around week 10) has a beating heart, a developing brain, limb buds that become arms and legs, and the beginnings of every major organ system. This is also the period of highest vulnerability to disruptions in development, which is why early prenatal care focuses heavily on nutrition, avoiding harmful substances, and screening for chromosomal conditions.
For the pregnant person, the first trimester often brings nausea, fatigue, and breast tenderness as hormone levels surge. Many people feel the worst during this stretch even though the fetus is still tiny.
Second Trimester: Rapid Growth
The second trimester is a period of rapid growth and development. The fetus goes from roughly 3 inches long at week 14 to over a foot long by the end of week 27. Bones harden, muscles strengthen, and the fetus begins moving in ways you can feel, usually between weeks 18 and 22. Facial features become distinct, fingerprints form, and the fetus starts responding to sound.
Many people find the second trimester the most comfortable. Early pregnancy symptoms tend to ease, energy returns, and the pregnancy becomes visible without yet being physically burdensome.
Third Trimester: Weight Gain and Maturation
The third trimester is about putting on weight and getting organs ready to function outside the womb. The fetus’s lungs mature, the brain develops rapidly, and a layer of fat builds under the skin that will help regulate body temperature after birth. The fetus roughly doubles or triples in weight during this final stretch, reaching an average of 6 to 9 pounds by the time of delivery.
For you, this trimester brings increasing physical strain: back pain, shortness of breath, frequent urination, and difficulty sleeping as the uterus takes up more space. Braxton Hicks contractions (practice contractions) become more common in the final weeks.
When “Full Term” Actually Starts
Reaching the third trimester doesn’t mean the baby is ready to be born. ACOG breaks the final weeks into more specific 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 outcomes improve with each additional week between 37 and 39 weeks. A baby born at 37 weeks is technically “term” but has higher rates of breathing problems and feeding difficulties than one born at 39 weeks. This is why providers generally avoid elective inductions or scheduled deliveries before 39 weeks unless there’s a medical reason.
Your Due Date Is an Estimate
The 40-week framework creates a specific due date, but only about 4% of babies arrive on that exact day. Around 70% are born within 10 days of the estimated date. Natural variation in pregnancy length spans a surprisingly wide range. One NIH study found that even after pinpointing the exact day of ovulation, the length of pregnancy varied by as much as five weeks among healthy women with uncomplicated pregnancies.
So while trimester divisions give you and your provider a useful roadmap, every pregnancy has its own timeline. The week markers are guideposts, not deadlines.
The “Fourth Trimester”
You may also hear about a fourth trimester, which refers to the first 12 weeks after delivery. This isn’t an official medical term in the same way the other three trimesters are, but it’s gaining traction among clinicians because the postpartum period involves significant physiological changes. During pregnancy, your cardiac output increases by up to 45% and your heart rate rises by 20 to 25%. After delivery, all of those shifts reverse, and the body needs weeks to recalibrate. The fourth trimester framework encourages closer monitoring and support during this recovery window, rather than treating birth as the finish line.