Pregnancy is divided into three trimesters, each lasting roughly 13 weeks across a full 40-week pregnancy. The first trimester covers weeks 1 through 12, the second spans weeks 13 through 27, and the third runs from week 28 until delivery. Each trimester brings distinct changes for both the developing baby and your body, with different symptoms, milestones, and screening tests along the way.
First Trimester: Weeks 1 Through 12
The first trimester is when the most dramatic development happens at the smallest scale. By the time most people even confirm a pregnancy (around weeks 4 to 6), the foundations for the brain, spinal cord, heart, and digestive system are already forming. Between weeks 9 and 12, cardiac tissue starts developing, webbed fingers and toes poke out from tiny hands and feet, and cartilage begins forming in the limbs, though it won’t harden into bone for several more weeks.
Behind the scenes, your body ramps up hormone production to sustain the pregnancy. The placenta produces a hormone called hCG, which signals your body to stop menstruating and triggers a surge in progesterone and estrogen. Progesterone thickens the uterine lining and keeps the uterus relaxed, while estrogen supports fetal organ development and increases blood flow to the placenta. hCG peaks around week 10 and then gradually declines until birth. That peak is largely responsible for the worst of first-trimester nausea.
Calorie needs don’t actually increase much during this stage. The recommendation for most people with a normal pre-pregnancy weight is about 1,800 calories per day in the first trimester, roughly the same as before pregnancy. What matters more now is getting enough folate, iron, and fluids, especially if nausea makes eating difficult.
Common First-Trimester Symptoms
Fatigue, breast tenderness, nausea (with or without vomiting), and frequent urination are the hallmarks of the first trimester. These are driven almost entirely by the rapid hormonal shifts happening in your body. Most people find that nausea eases significantly by weeks 12 to 14 as hCG levels drop from their peak.
Second Trimester: Weeks 13 Through 27
The second trimester is often called the most comfortable stretch of pregnancy. First-trimester nausea typically fades, energy returns, and the baby is growing quickly but isn’t yet large enough to cause the physical strain that comes later. This is when the pregnancy starts to feel real in a new way: you can see it and, eventually, feel it.
The biggest milestone of the second trimester is quickening, the first time you feel the baby move. If you’ve been pregnant before, you may notice fluttering as early as 16 weeks. For a first pregnancy, it’s common not to feel movement until around 20 weeks. Those early movements are subtle, often described as bubbles or a light tapping, and become stronger and more distinct as the weeks progress.
The Anatomy Scan
Between 18 and 22 weeks, you’ll typically have a detailed ultrasound sometimes called the anatomy scan or anomaly scan. This is the most thorough imaging of the pregnancy, checking the baby’s organs, limbs, spine, brain, and heart. It’s also often the appointment where parents can learn the baby’s sex, if they want to know.
Glucose Screening
Between 24 and 28 weeks, most people are screened for gestational diabetes with a glucose test. You drink a sugary solution and have your blood drawn about an hour later to see how your body processes the sugar. If you have risk factors for diabetes or had gestational diabetes in a previous pregnancy, this test may be done earlier in the first trimester instead.
Calorie and Nutrition Shifts
Your calorie needs rise in the second trimester to about 2,200 per day, roughly 300 to 400 more than the first trimester. This is when the baby’s growth accelerates and your blood volume expands significantly. Iron becomes especially important now because your body is producing far more blood than usual.
Third Trimester: Weeks 28 Through 40
The third trimester is about the baby gaining weight, maturing its organs, and getting into position for delivery. The lungs are one of the last organs to fully mature, which is one reason why premature birth carries risk. By week 36, most babies have turned head down in preparation for birth. If yours hasn’t, your provider may discuss options for encouraging the baby to turn, typically around week 37.
Daily calorie needs peak at about 2,400 per day in the third trimester. The baby is putting on fat rapidly during these final weeks, gaining roughly half a pound per week in the last month.
Practice Contractions
You may start feeling Braxton Hicks contractions, a tightening sensation across your abdomen that can begin as early as the second trimester but becomes more noticeable in the third. These are not labor. They’re irregular, don’t increase in intensity, and usually stop when you change positions or drink water. Real labor contractions grow closer together, last longer, and get progressively more painful.
Common Third-Trimester Symptoms
As the baby takes up more space, your lungs have less room to expand and your bladder is under constant pressure. Shortness of breath, frequent urination, back pain, swollen feet, and difficulty sleeping are all typical. Heartburn often worsens because the growing uterus pushes stomach contents upward. Many of these symptoms ease noticeably once the baby “drops” lower into the pelvis, which can happen a few weeks before labor in a first pregnancy or not until labor itself in subsequent pregnancies.
When Is a Baby Considered Full Term?
Not all deliveries at 37 weeks or later are the same. The American College of Obstetricians and Gynecologists breaks it down into specific categories:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Post-term: 42 weeks and beyond
Babies born at 39 weeks or later generally have better outcomes than those born at 37 or 38 weeks, even though both fall within what people casually call “term.” Those last two weeks allow for continued brain development and lung maturation that make a measurable difference. This distinction is why elective deliveries are typically not scheduled before 39 weeks unless there’s a medical reason.