Pregnancy is divided into three trimesters, each roughly 13 to 14 weeks long, spanning a total of about 40 weeks. The first trimester runs from the first day of your last menstrual period through 13 weeks and 6 days. The second trimester covers weeks 14 through 27. The third trimester spans weeks 28 through 40. These divisions aren’t arbitrary. Each trimester reflects a distinct phase of fetal development and brings its own set of physical changes for you.
Why Pregnancy Starts Before Conception
One detail that confuses almost everyone: pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you actually conceived. Conception typically happens 11 to 21 days after the start of your last period, which means you’re already considered “two weeks pregnant” or more by the time a fertilized egg implants. This system exists because most people can reliably recall when their period started, while pinpointing the exact day of conception is much harder.
Early ultrasounds can fine-tune your due date by measuring the embryo’s length. In the first trimester, this measurement is accurate to within 5 to 7 days. That precision decreases as pregnancy progresses, which is why early dating scans matter.
First Trimester: Weeks 1 Through 13
The first trimester is when all major organ systems form. By the end of this stretch, a developing embryo has gone from a cluster of cells to a fetus with a beating heart, forming limbs, and early versions of every major organ. ACOG describes it as the period of “fertilization and major organ development,” and it’s the phase where the pregnancy is most vulnerable to disruption.
Hormonally, your body is working overtime. Human chorionic gonadotropin (hCG), the hormone that triggers a positive pregnancy test, rises rapidly and peaks around week 10 before gradually declining. That surge is a big reason behind first-trimester nausea, fatigue, and breast tenderness. Many people feel their worst during weeks 6 through 10, right when hCG levels are climbing fastest.
By the end of week 13, the fetus is about 3 inches long. The placenta has taken over hormone production from the ovaries, a transition that often brings some relief from early symptoms as you cross into the second trimester.
Second Trimester: Weeks 14 Through 27
The second trimester is often called the most comfortable stretch of pregnancy, and there’s a biological reason for that. The hormonal upheaval of the first trimester has settled, nausea typically fades, and energy levels rebound. Meanwhile, you’re not yet large enough for the physical discomforts of late pregnancy to take hold.
For the fetus, this is a period of rapid growth. Bones harden, facial features become distinct, and the nervous system matures enough to allow coordinated movement. Around 18 weeks after conception (roughly week 20 of pregnancy), many people feel fetal movement for the first time, a sensation traditionally called “quickening.” It often starts as a subtle fluttering that’s easy to mistake for gas, becoming more obvious over the following weeks.
A significant medical milestone falls near the end of this trimester. Around week 23, some babies born prematurely may be able to survive outside the uterus with intensive medical support. This threshold, known as the age of viability, has shifted earlier over the decades thanks to advances in neonatal care, but outcomes at this stage remain uncertain and vary widely.
Third Trimester: Weeks 28 Through 40
The final trimester is all about weight gain and organ maturation. The fetus’s lungs develop the ability to breathe air, the brain grows rapidly, and fat accumulates under the skin to help regulate body temperature after birth. ACOG summarizes it as the time when “the fetus’s weight increases and the organs mature so they will be ready to function after birth.”
For you, this trimester brings its own challenges. The growing uterus presses on your bladder, diaphragm, and lower back, which can mean frequent urination, shortness of breath, and back pain. Sleep often becomes difficult. Braxton-Hicks contractions, irregular tightening of the uterus that aren’t true labor, become more common as your body prepares for delivery.
A full-term pregnancy is considered 39 to 40 weeks, though anything from 37 weeks onward is classified as “early term.” Babies born before 37 weeks are premature, and their outcomes depend heavily on how far into the third trimester they’ve progressed.
Why Sources Sometimes Disagree on the Cutoffs
If you’ve noticed different websites listing slightly different week ranges, you’re not imagining it. ACOG places the second trimester start at 14 weeks and 0 days, while Cleveland Clinic and some other sources round down and say it begins “around week 13.” The difference is minor, just a few days, and stems from whether the source counts completed weeks or the week you’re currently in. The biological changes happening in your body don’t flip like a switch at any specific date, so a few days’ difference in the cutoff is clinically meaningless.
The Fourth Trimester
Though not an official medical term, the “fourth trimester” refers to the first 12 weeks after giving birth. It’s increasingly recognized as a critical period for both the newborn and the mother. Your body is recovering from delivery, hormone levels are shifting dramatically again, and the demands of caring for a newborn can affect sleep, mental health, and physical healing. Columbia University’s medical center describes those 12 weeks as “just as important for a mother’s health as the first three trimesters,” a framing that reflects growing awareness that postpartum care deserves the same attention as prenatal care.