Pregnancy lasts about 40 weeks from the first day of your last menstrual period, and every week brings measurable changes to both your body and the developing baby. In the earliest weeks, a cluster of cells smaller than a poppy seed begins forming a brain and heart. By the end, you’re carrying a full-term baby whose lungs can breathe independently. Here’s what’s happening at each stage.
Weeks 1 Through 4: Before You Know
Weeks 1 and 2 are counted from your last period, so conception itself typically happens around week 2 or 3. By the time a fertilized egg implants in the uterine wall (around day 6 to 10 after fertilization), the hormone hCG starts rising. At week 3, hCG levels may only be 5 to 50 mIU/mL, which is why most home tests can’t pick up a pregnancy this early.
By the end of week 4, the embryo is about 2 millimeters long, roughly the size of a poppy seed. Structures that will become the brain and spinal cord (called the neural tube) are already forming. hCG levels climb rapidly, nearly doubling every three days for the next several weeks. That hormonal surge is what triggers the earliest symptoms: breast tenderness, fatigue, and sometimes nausea before you’ve even missed a period.
Weeks 5 Through 8: A Heartbeat and Early Organs
This stretch is when the embryo’s major organs begin taking shape. Around weeks 5 and 6, cells that form the heart start clustering together and can produce a detectable pulse. By about 6 weeks, tiny limb buds appear where arms and legs will grow. On an early ultrasound, the embryonic heart rate starts at roughly 110 beats per minute at 6 weeks and climbs to about 159 bpm by week 8. A rate below 100 bpm before 6.3 weeks can signal a higher risk of early loss.
By the end of week 8, the embryo’s eyes, mouth, and the foundations of nearly every organ system have formed. It’s still tiny, but the basic blueprint of a human body is in place. hCG levels peak somewhere between weeks 8 and 10, reaching as high as 229,000 mIU/mL. This peak is often when nausea and vomiting hit their worst. If morning sickness has been mild so far, it may intensify during these weeks.
Weeks 9 Through 12: Transition to Fetus
Week 9 marks the shift from embryo to fetus, meaning the baby’s basic structures are now refining rather than forming from scratch. Biological sex is determined around week 9, though it’s far too early to see on ultrasound. Fingers and toes separate, and the face becomes more recognizably human.
Between weeks 11 and 13, you’ll likely be offered a nuchal translucency ultrasound. This scan measures a small fluid-filled space at the back of the baby’s neck and screens for chromosomal conditions like Down syndrome. A blood draw for first-trimester genetic screening typically happens around the same time. Cell-free DNA testing (sometimes called NIPT) can also be done starting around week 10, which screens for common chromosomal abnormalities with high accuracy.
For many people, nausea begins to ease toward the end of this stretch as hCG levels plateau and then decline slightly. Fatigue often lifts too, which is why the second trimester has a reputation as the most comfortable phase.
Weeks 13 Through 16: The Second Trimester Begins
Your blood volume is already climbing by now and will eventually increase by about 45% above your pre-pregnancy level. That’s an extra 1,200 to 1,600 milliliters of blood circulating through your body. This expansion supports the placenta and growing baby, but it can also cause nasal congestion, visible veins, and occasional dizziness when you stand up quickly.
The baby is growing fast. By week 14, it can make facial expressions like squinting. By week 16, the body is more proportional, and the skeleton is hardening from soft cartilage into bone. If you’ve been pregnant before, you may start feeling faint fluttering movements (called quickening) as early as week 16. First-time parents usually don’t notice these until closer to week 20.
Weeks 17 Through 20: The Anatomy Scan
Around week 18 to 20, you’ll have what’s often the most detailed ultrasound of the entire pregnancy: the anatomy scan. A specialist examines the baby’s brain, heart, spine, kidneys, limbs, and other structures to check for any developmental concerns. This is also when most parents learn the baby’s sex if they want to know.
The baby is now swallowing amniotic fluid, and a waxy coating called vernix begins forming over the skin. You’ll likely feel definitive kicks and rolls by week 20 if you haven’t already. These movements start subtle, like bubbles or gentle taps, and grow stronger over the coming weeks.
Weeks 21 Through 24: A Critical Threshold
These weeks mark a major milestone in lung development. Tube-like structures form in the lungs during weeks 22 through 25, but the specialized cells that produce surfactant (a substance the lungs need to inflate properly after birth) don’t appear until around week 26. Without surfactant, a baby’s lungs can’t function on their own.
This is why viability depends heavily on gestational age. Babies born before 24 weeks survive about 24% of the time. At a completed 24 weeks, survival jumps to roughly 72%, though outcomes vary widely depending on the hospital’s resources. Each additional day in the womb during this window meaningfully improves the odds.
Between weeks 24 and 28, you’ll be offered a glucose tolerance test to screen for gestational diabetes. You’ll drink a sugary solution and have your blood drawn to see how your body processes the sugar. If results are elevated, a longer follow-up test confirms the diagnosis.
Weeks 25 Through 28: Rapid Brain Growth
The baby’s brain is developing quickly now, forming the grooves and folds that characterize a mature brain. Eyes can open and respond to light. The baby settles into more predictable sleep and wake cycles, which you’ll notice as patterns in the kicking.
Surfactant-producing cells in the lungs increase dramatically around week 28, jumping from a handful of cells to significantly higher numbers. This is a turning point for lung readiness, though the lungs still need weeks more to mature fully. Your body is working harder too. Most of the 50% increase in plasma volume happens by around week 34, but the ramp-up is steep now. You may notice more swelling in your feet and ankles, and shortness of breath becomes common as your uterus pushes upward against your diaphragm.
Weeks 29 Through 33: Gaining Weight Fast
The baby is packing on fat, gaining roughly half a pound per week during this stretch. This fat layer helps regulate body temperature after birth. The bones are fully developed but still soft and flexible, which matters for fitting through the birth canal later.
You may start noticing Braxton Hicks contractions, sometimes called practice contractions. These tightenings of the uterus can be uncomfortable or even painful, but they’re irregular, don’t follow a pattern, and typically stop if you rest or drink water. True labor contractions, by contrast, come at regular intervals, get progressively closer together, and last about 60 to 90 seconds each. If resting makes your contractions stop, they aren’t the real thing.
Weeks 34 Through 37: Lungs Approaching Maturity
Week 34 is a landmark for the lungs. Surfactant now spreads throughout the air sacs, meaning the lungs can inflate and exchange oxygen far more effectively. Babies born at 34 weeks still often need some respiratory support, but their outcomes are dramatically better than those born even a few weeks earlier.
The baby may settle into a head-down position around this time, though some don’t flip until later (or at all). Your provider will start checking the baby’s position at appointments. Prenatal visits become more frequent now, often weekly or biweekly, and may include checking your cervix for early signs of dilation.
Plasma volume peaks around week 34. You’re carrying roughly 50 to 60% more fluid in your bloodstream than before pregnancy. This can make rings feel tight, shoes feel small, and your face look slightly fuller.
Weeks 38 Through 40: Full Term
A pregnancy is considered “early term” at 37 weeks and “full term” at 39 weeks. The final weeks aren’t just waiting. The baby’s brain and lungs are still maturing, and the thin walls of the air sacs in the lungs are reaching their adult-like structure between weeks 40 and 44. This is why elective deliveries before 39 weeks are discouraged unless medically necessary.
You’ll want to know the difference between Braxton Hicks and real labor contractions during these weeks. True labor contractions come in a predictable rhythm, don’t go away with rest or hydration, and intensify over time. Other signs that labor is approaching include a mucus plug discharge, a feeling of the baby dropping lower into your pelvis, and sometimes a gush or trickle of fluid if your water breaks.
The average baby at 40 weeks weighs about 7 to 8 pounds and measures around 20 inches. But healthy full-term babies vary considerably. What matters most is consistent growth along the baby’s own curve, which your provider has been tracking through fundal height measurements and ultrasounds throughout the pregnancy.