Pregnancy lasts about 40 weeks from the first day of your last period, and every week brings measurable changes in both your baby’s development and your own body. Your baby goes from a cluster of cells smaller than a poppy seed to a full-term newborn averaging 6 to 9 pounds and 18 to 20 inches long. Here’s what happens along the way.
Weeks 1 Through 4: Conception and Implantation
The first two weeks of pregnancy are counted from the start of your last menstrual period, so conception typically happens around week 2 or 3. By the end of week 4, the fertilized egg has implanted in the uterine wall and begun dividing rapidly. At this point, a pregnancy hormone called hCG starts rising. Your levels sit around 100 IU/L near the time of your expected (but missed) period, which is just enough for a home test to detect.
Most people feel nothing unusual during these first weeks. Some notice light spotting or mild cramping as the embryo implants, which is easy to mistake for an early period.
Weeks 5 Through 8: Organs Begin Forming
This is when things accelerate. The embryo’s heart starts beating, the neural tube (which becomes the brain and spinal cord) closes, and tiny limb buds appear. By week 8, all major organs and body systems are developing, and the placenta is working. Your baby is still tiny, though, measuring less than half an inch at week 9.
For you, this stretch is often the roughest. hCG levels climb steeply, peaking between weeks 8 and 10 at roughly 100,000 IU/L, about a thousand times higher than at your missed period. That hormonal surge is a key driver of morning sickness, which despite its name can hit at any hour. Breast tenderness, fatigue, and frequent urination are also common. Progesterone, produced by a structure in the ovary called the corpus luteum, keeps the pregnancy stable during these early weeks until the placenta takes over around week 10.
Weeks 9 Through 12: End of the First Trimester
By week 12, your baby is about 2 inches long and weighs roughly half an ounce. Fingers and toes have formed, and facial features are becoming more distinct. The risk of miscarriage drops significantly after this milestone, which is why many people choose this point to share the news.
Your first prenatal visit usually happens between weeks 8 and 12. It typically includes bloodwork, a urine screen, and often an ultrasound to confirm the due date and check for a heartbeat. You may also be offered first-trimester screening for chromosomal conditions, which combines a blood test with an ultrasound measurement of the fluid at the back of the baby’s neck.
Weeks 13 Through 16: Energy Returns
Welcome to the second trimester. Most people find that morning sickness eases around weeks 13 or 14 and their energy picks up noticeably. Your body is adjusting to the higher hormone levels rather than still ramping up, which makes a real difference in how you feel day to day.
Your baby is growing fast. By week 16, they’re about 5 inches long and 5 ounces, with developed lips and ears sensitive enough to pick up sound. This is also when many providers offer a blood screening (sometimes called a quad screen) or cell-free DNA testing to check for certain genetic conditions.
Weeks 17 Through 20: Movement and the Anatomy Scan
Around week 20, the halfway mark, you may start feeling your baby move. These first movements, called quickening, often feel like a flutter or gentle bubbling. First-time parents sometimes don’t recognize them until a week or two later, while people who’ve been pregnant before may notice them earlier.
The anatomy scan ultrasound is typically scheduled between weeks 18 and 22. This detailed scan checks the baby’s brain, heart, spine, kidneys, and limbs, and it’s usually when you can find out the sex if you want to know. Your baby is now about 10 inches long and weighs around 1 pound. Hearing is developing, and by week 22 your baby can hear your heartbeat, your breathing, and your stomach rumbling.
Weeks 21 Through 27: Rapid Growth
Your baby’s features are becoming more refined. Around week 23, ridges form in the palms and soles that will become unique fingerprints and footprints. Rapid eye movements begin. The lungs start producing surfactant, a substance that will allow the air sacs to inflate and prevent the lungs from collapsing after birth. This is a critical step toward being able to breathe independently.
By week 24, your baby measures about 12 inches and weighs a little over 1 pound. At week 28, they’ve reached roughly 14 inches and 2.5 pounds. You’ll likely notice the kicks and rolls getting stronger and more regular. Some providers recommend starting kick counts around week 28: tracking how long it takes to feel 10 movements, which usually happens within two hours when your baby is active.
For your body, the second trimester can bring round ligament pain (sharp twinges on the sides of your lower belly as the uterus stretches), leg cramps, and nasal congestion from increased blood flow. Weight gain picks up during the second and third trimesters. If you started pregnancy at a normal BMI, the recommended pace is about 1 pound per week during this stretch, with a total gain of 25 to 35 pounds by delivery.
Weight Gain Guidelines by Pre-Pregnancy BMI
How much weight you’re expected to gain depends on where you started:
- Underweight (BMI under 18.5): 28 to 40 pounds total
- Normal weight (BMI 18.5 to 24.9): 25 to 35 pounds total
- Overweight (BMI 25 to 29.9): 15 to 25 pounds total
- Obese (BMI 30 or higher): 11 to 20 pounds total
Most of the gain happens in the second and third trimesters. In the first trimester, a gain of 1 to 4.5 pounds is typical regardless of your starting weight.
Weeks 28 Through 31: The Third Trimester Begins
Your baby’s eyelids can now partially open at week 28, and the nervous system has matured enough to regulate body temperature. By week 30, the eyes open wide and red blood cells are being produced in the bone marrow rather than in earlier temporary sites. At week 31, most major development is complete. From here on, the primary job is gaining weight.
You’ll start seeing your provider more often, typically every two weeks and then weekly after week 36. Prenatal visits now focus on your blood pressure, the baby’s position, and your baby’s growth. Between weeks 24 and 28 you’ll take a glucose screening test (drinking a sweet solution and having your blood drawn) to check for gestational diabetes. If your blood type is Rh-negative, you’ll receive a shot around week 28 to prevent complications.
Braxton Hicks contractions often become noticeable in the third trimester. These are irregular tightening sensations across your belly that come and go without a pattern. They’re your uterus “practicing” and are generally painless, though they can be uncomfortable. Unlike true labor contractions, they don’t get closer together or stronger over time.
Weeks 32 Through 36: Gaining Weight Fast
Your baby is packing on fat rapidly. By week 32, they measure about 18 inches and weigh around 5 pounds. The bones are hardening, though the skull stays flexible and soft to allow passage through the birth canal. By week 36, your baby weighs 6 to 7 pounds and the skin is smoothing out as fat fills in underneath.
Between weeks 35 and 37, you’ll be screened for Group B Streptococcus (GBS), a common bacterium that about 25% of healthy women carry. The test is a simple swab. If you test positive, you’ll receive antibiotics during labor to protect the baby. About 6% of carriers go undetected by screening, but the test catches the vast majority.
This is also when your baby typically moves into a head-down position. If they haven’t turned by week 36, your provider may discuss options. You might feel increased pressure on your bladder, heartburn, and shortness of breath as the baby takes up more space. Once the baby “drops” lower into the pelvis (called lightening), breathing gets easier but bathroom trips increase.
Weeks 37 Through 40: Full Term
A pregnancy is considered early term at 37 weeks and full term at 39 weeks. During these final weeks, fat continues to accumulate all over your baby’s body for warmth after birth. Lung development reaches full maturity, and the brain is adding critical connections that support breathing, feeding, and temperature regulation.
At 40 weeks, the average baby is 18 to 20 inches long and 6 to 9 pounds, though healthy birth weights vary widely. Progesterone levels at this stage reach 100 to 200 ng/ml, and the placenta is producing about 250 mg of progesterone per day to maintain the pregnancy until labor begins.
Many first-time parents deliver a few days after their due date. If you reach 41 weeks without labor starting, your provider will discuss induction. In the meantime, the signs that labor is approaching include the loss of the mucus plug, a “bloody show” (pink or blood-tinged discharge), regular contractions that get progressively stronger and closer together, and your water breaking. True labor contractions build in intensity, last 30 to 70 seconds, and don’t stop when you change positions.
Nutrition Basics Throughout Pregnancy
Folic acid is the single most important supplement in early pregnancy, helping prevent neural tube defects during weeks 3 through 5 when the spinal cord is forming. Most prenatal vitamins supply 400 to 800 micrograms. You also need adequate vitamin D. The recommended daily intake during pregnancy is 600 international units, though most prenatal vitamins contain only 400 IU per tablet. If you’re found to be deficient, supplementing with 1,000 to 2,000 IU daily is considered safe, with an upper limit of 4,000 IU per day.
Iron needs increase substantially because your blood volume expands by nearly 50% over the course of pregnancy. Iron deficiency is one of the most common nutritional gaps, and it can leave you feeling exhausted beyond normal pregnancy fatigue. Iodine is another nutrient that’s easy to miss if your diet is low in dairy or seafood, and it’s essential for your baby’s brain development. A prenatal vitamin that includes both iron and iodine covers the basics, but your provider may adjust doses based on your bloodwork.