Pregnancy lasts about 40 weeks from the first day of your last period and brings dramatic changes to your body in every trimester. Some of those changes are obvious, like a growing belly, while others are invisible shifts in your heart rate, blood sugar regulation, and hormones that quietly reshape how your body functions for months. Here’s what actually happens, trimester by trimester, and what you can do to stay ahead of it.
First Trimester: Weeks 1 Through 13
The first trimester is when you’ll likely feel the most different relative to how little you’re showing. By week 4, fatigue and nausea often kick in, driven largely by a hormone called hCG that your placenta starts producing almost immediately after conception. hCG nearly doubles every three days in early pregnancy and peaks around week 10. It signals your body to stop menstruating and ramp up production of progesterone and estrogen, the two hormones that will sustain the pregnancy from here on out.
That hormonal surge is behind most of what you feel in these early weeks. Between weeks 4 and 6, expect breast tenderness, bloating, frequent urination, and waves of nausea that can strike at any time of day. By week 6, mood swings and temperature fluctuations are common. Your heart is already working harder: cardiac output increases by about 20% by week 8, which can make you feel winded doing things that never bothered you before. Around week 9, heartburn and extreme fatigue tend to intensify. By weeks 11 and 12, some women notice acne, visible breast changes, early weight gain, and the so-called “pregnancy glow” from increased blood flow to the skin.
Meanwhile, your baby is undergoing the most critical developmental window of the entire pregnancy. All major organs begin forming during the first trimester. This is why prenatal vitamins with folic acid matter most in these early weeks, often before you even know you’re pregnant.
Second Trimester: Weeks 14 Through 27
Many women feel their best during the second trimester. The nausea that dominated the first 12 weeks typically fades as hCG levels decline. Your blood pressure actually drops during this period before rising again later, which can occasionally cause lightheadedness when you stand up quickly.
Under the surface, your body is making significant metabolic adjustments. Insulin resistance begins developing in the second trimester, meaning your cells don’t respond to insulin as efficiently as usual. This is a normal adaptation that keeps extra glucose available for your growing baby, but it’s also why gestational diabetes screening happens between weeks 24 and 28. The test involves drinking a glucose solution and having your blood sugar measured afterward. If the result comes back elevated (above 140 mg/dl at two hours is a common threshold), your provider will do a follow-up test to confirm a diagnosis.
Your baby is growing rapidly during these weeks. This is when you’ll start feeling movement, typically between weeks 16 and 22 for a first pregnancy. Your cardiac output hits its maximum between weeks 20 and 28, and you’ll notice your belly growing noticeably as the uterus rises above your belly button.
Third Trimester: Weeks 28 Through 40
The final stretch is largely about your baby gaining weight and its organs maturing enough to function independently after birth. For you, the increasing size of your uterus puts pressure on your bladder, lungs, and lower back, which translates to more frequent bathroom trips, shortness of breath, and back pain. Your heart rate stays elevated by 10 to 20 beats per minute above your pre-pregnancy baseline to maintain the higher blood volume your body now carries. Blood pressure rises back to pre-pregnancy levels. Insulin resistance peaks, so blood sugar management matters most now.
Sleep becomes harder to come by. Between the physical discomfort, the need to urinate at night, and difficulty finding a comfortable position, broken sleep is nearly universal in the third trimester. Swelling in your feet and ankles is also common due to increased fluid retention.
Braxton Hicks vs. Real Labor
Starting in the third trimester (and sometimes earlier), you may notice your uterus tightening and then releasing. These are Braxton Hicks contractions, essentially practice contractions. They feel like mild menstrual cramps or a firm tightening in one area of your abdomen. They’re irregular, unpredictable, and usually painless. Changing positions or going for a walk often makes them stop.
Real labor contractions are different in three specific ways. First, they follow a regular pattern and gradually get closer together, lasting 30 to 70 seconds each. They don’t stop when you change positions. Second, they start in your lower back and wrap around to the front of your abdomen, or vice versa, rather than staying focused in one spot. Third, they get progressively stronger over time to the point where talking or moving becomes difficult. If your contractions follow that pattern, labor is likely underway.
How Much Weight You Should Gain
Weight gain recommendations depend on your pre-pregnancy BMI. For a single pregnancy, the CDC guidelines break down like this:
- Underweight (BMI under 18.5): 28 to 40 pounds
- Normal weight (BMI 18.5 to 24.9): 25 to 35 pounds
- Overweight (BMI 25.0 to 29.9): 15 to 25 pounds
- Obese (BMI 30.0 to 39.9): 11 to 20 pounds
For twins, the numbers are significantly higher. Women at a normal weight carrying twins are advised to gain 37 to 54 pounds. Most weight gain happens in the second and third trimesters, so don’t worry if the scale barely moves in the first 12 weeks.
Foods and Substances to Avoid
Pregnancy suppresses parts of your immune system, making you more vulnerable to foodborne infections like listeria and toxoplasmosis. Both can cause serious complications for the baby. The foods that carry the highest risk fall into a few categories:
- Deli meats and hot dogs: Unless heated until steaming, these can harbor listeria. The same goes for refrigerated pâté and meat spreads.
- Raw or undercooked meat, poultry, and eggs: This includes runny eggs, rare steak, homemade Caesar dressing, raw cookie dough, and homemade eggnog.
- Unpasteurized dairy: Raw milk and soft cheeses made from it (queso fresco, brie, camembert, blue cheese) are risky. Even pasteurized queso fresco-type cheeses can be a concern.
- High-mercury fish: Shark, swordfish, king mackerel, and tilefish accumulate mercury levels that can affect fetal brain development.
- Unwashed produce and raw sprouts: Alfalfa and bean sprouts are particularly prone to contamination. Wash all fruits and vegetables before eating.
- Unpasteurized juice or cider: These can carry harmful bacteria.
Premade deli salads (tuna salad, chicken salad, egg salad, coleslaw) from a grocery deli counter are also considered riskier because of how they’re handled and stored.
Mental Health During Pregnancy
Mood swings driven by hormonal changes are one thing. Persistent depression or anxiety is something different, and it’s far more common than most people realize. An estimated 15 to 21% of pregnant and postpartum women experience a perinatal mood or anxiety disorder, making it the single most common complication of pregnancy. About 1 in 7 women develop perinatal depression specifically, and 13 to 21% experience perinatal anxiety.
These aren’t the same as occasional sadness or worry about becoming a parent. Warning signs include feeling hopeless or numb for weeks at a time, losing interest in things you normally enjoy, persistent difficulty sleeping that goes beyond physical discomfort, excessive worry that you can’t control, or thoughts of harming yourself. Women who had depression or anxiety before pregnancy are roughly three and a half times more likely to develop these conditions during or after pregnancy. A history of adverse childhood experiences also increases risk. If what you’re feeling goes beyond normal pregnancy ups and downs, it’s a treatable condition, not a personal failing.
Prenatal Visits and Screening Tests
Prenatal care typically follows a pattern: monthly visits through about week 28, every two weeks until week 36, then weekly until delivery. The exact schedule varies depending on your health and risk factors.
Several key screenings happen at specific windows. In the first trimester, bloodwork and an ultrasound between weeks 11 and 14 can screen for chromosomal conditions. The gestational diabetes glucose test happens between weeks 24 and 28. Your provider will check your blood pressure, urine, and the baby’s heart rate at every visit. In the third trimester, you’ll be tested for Group B strep, a common bacterium that’s harmless to you but can be dangerous for the baby during delivery if untreated. You’ll also have ultrasounds to check the baby’s position as your due date approaches.
Throughout all of this, the practical reality is that pregnancy is 40 weeks of constant change. What bothers you in one trimester often resolves in the next, only to be replaced by something new. Knowing the general timeline helps you tell the difference between a normal shift and something that deserves attention.