Pregnancy is typically divided into three trimesters, each with its own developmental milestones, medical tests, and physical symptoms. Understanding what to expect at each stage makes the process less stressful and helps you know what to discuss with your provider. Here is a practical walk-through.

First trimester (weeks 1-13)

Weeks 1-4: foundations

By convention, weeks 1-2 are before conception (counted from the last menstrual period). Conception happens around week 2-3. Implantation occurs around week 4, when the embryo attaches to the uterine lining.

Home pregnancy tests typically turn positive around week 4 because that is when human chorionic gonadotropin (hCG) rises high enough to be detected in urine.

Weeks 5-8: major organ formation

This is when the heart starts beating (around week 6), the neural tube forms (becoming brain and spine), and all major organ systems begin developing. The embryo grows from about 2mm at week 5 to 16mm at week 8.

This is also when morning sickness peaks for many — typically nausea and food aversions starting around week 6-7. About 70-80% of pregnant people experience some degree of nausea in the first trimester. Tools that help: small frequent meals, protein at breakfast, ginger, and prenatal vitamins taken at bedtime.

Weeks 9-12: looking like a baby

The embryo becomes a fetus at week 9. Fingers and toes separate. Facial features become recognizable. By week 12 the fetus is about 2.5 inches long and has all the major organs present — though they will continue to develop and mature for months.

First trimester medical milestones

  • First prenatal visit: typically 8-10 weeks. Blood work, urine sample, genetic screening discussion, pelvic exam, and a confirmation ultrasound.
  • First trimester screening (weeks 11-13): nuchal translucency ultrasound + blood work to screen for chromosomal conditions (Down syndrome, trisomy 18).
  • Non-invasive prenatal testing (NIPT): blood test from 10 weeks onward, screens for chromosomal conditions with high accuracy. Also reveals the sex of the fetus if desired.

Miscarriage risk drops meaningfully after the first trimester — from about 10-20% in the first trimester to about 1-2% thereafter.

Second trimester (weeks 14-27)

For many people, this is the best trimester. Nausea usually subsides by week 14-16. Energy returns. The belly shows clearly (“the bump” becomes visible around week 16-20 for most first pregnancies). You can feel the baby move. Sleep is still reasonably comfortable.

Weeks 14-17: growth acceleration

The fetus grows from 3 inches to about 5 inches. It starts developing fat. Hair appears on the head. The musculoskeletal system is working well enough that small movements begin.

Weeks 18-22: quickening and anatomy scan

Most people feel the first clear fetal movement (called “quickening”) between 18 and 22 weeks, though it can happen as early as 16 or as late as 24. First pregnancies tend to feel movement later than subsequent pregnancies.

The 20-week anatomy scan is a comprehensive ultrasound that checks every major organ system, measures fetal growth, and confirms the placenta position. This is often when the sex is revealed (if you want to know). It is also the scan most parents remember — high-resolution images of the baby’s profile, hands, feet, and face.

Weeks 23-27: viability threshold

By 24 weeks, a premature baby has a realistic chance of survival with intensive care. This is called the “viability threshold.” It continues to improve quickly — by 28 weeks, survival rates exceed 90% in developed NICU settings.

Fetal movements become stronger and more regular. Many pregnant people start feeling hiccups — rhythmic little pulses that last a few minutes at a time.

Second trimester medical milestones

  • Anatomy scan: ~20 weeks
  • Maternal serum screening (if not already done): between 15-22 weeks
  • Glucose challenge test: 24-28 weeks, screens for gestational diabetes
  • Rhogam shot (if Rh-negative): 28 weeks
  • Tdap vaccine (for pertussis protection): 27-36 weeks

Third trimester (weeks 28-40+)

The third trimester is when the baby focuses on growth and final development. From week 28 to week 40, the fetus typically gains about 4-5 pounds. Most of the organ development is complete; the work now is getting bigger and maturing systems (especially lungs and brain).

Weeks 28-32: prep for the outside world

Brain development accelerates. The fetus can open and close its eyes, distinguish light from dark, and react to sounds. Many parents report the baby responding to music, voices, or familiar sounds.

For pregnant people, physical symptoms often intensify. Heartburn, back pain, shortness of breath (from the uterus pushing up against the diaphragm), swelling in feet and ankles, and frequent urination are common. Sleep becomes harder as the belly grows.

Weeks 33-36: homestretch

The fetus is usually in the head-down position by week 36. If not, your provider may discuss options to encourage repositioning. Lungs are nearly mature; a baby born at this point usually does well with minimal NICU support.

Braxton Hicks contractions (practice contractions) may become more noticeable. Unlike real labor, they are irregular, do not intensify, and usually go away with position change or water.

Weeks 37-40: full term

At 39 weeks you are considered full term. The fetus is typically 6-9 pounds. Real labor can start any day. Signs of approaching labor include the mucus plug discharging, lightening (the baby dropping lower into the pelvis), and actual contractions that are regular, intensifying, and close together.

Third trimester medical milestones

  • Weekly checkups starting around 36 weeks
  • Group B strep test: 35-37 weeks
  • Cervical checks: starting late third trimester as needed
  • Labor and delivery planning discussions (birth plan, pain management, who will be present)

If the pregnancy extends past 41 weeks, induction is usually discussed. Beyond 42 weeks, induction is strongly recommended due to rising risks of placental insufficiency.

What to discuss with your provider

Each trimester has specific questions worth raising:

  • First trimester: medication safety (which of your current prescriptions are safe), prenatal vitamin choice, genetic testing options, nausea management, first symptoms worth calling about
  • Second trimester: kick count monitoring, warning signs of preterm labor, glucose test prep, pregnancy-safe exercise, when to start prenatal classes
  • Third trimester: birth plan, pain management preferences, feeding plans (breastfeeding support, formula choice), postpartum mental health resources, signs of labor vs false alarms

Warning signs at any stage

Call your provider immediately for:

  • Heavy bleeding
  • Severe abdominal pain
  • Severe headache or vision changes
  • Persistent swelling in hands or face (as opposed to just feet)
  • No fetal movement for 24 hours (after week 24)
  • Fever above 100.4°F
  • Symptoms of preeclampsia: high blood pressure, protein in urine
  • Rupture of membranes (water breaking) before 37 weeks

Track your weeks

Our pregnancy due date calculator gives you your current gestational age in weeks and days, so you always know which stage you are in. Pair it with a prenatal care schedule from your provider to stay on top of the medical milestones. The weeks go by faster than you think — and the week-by-week changes are one of the genuinely wondrous parts of the whole process.