Calculate Your Pregnancy Week

Week-by-Week Baby Development

Weeks 1-4
Weeks 5-8
Weeks 9-12
Weeks 13-16
Weeks 17-20
Weeks 21-24
Weeks 25-28
Weeks 29-32
Weeks 33-36
Weeks 37-40

Weeks 1-4: Conception & Implantation

Though you're not technically pregnant yet, your body is preparing for a baby. Your menstrual cycle is gearing up to release an egg, and your uterus is thickening to support a potential pregnancy.

  • Ovulation occurs around week 2
  • Fertilization happens when sperm meets egg
  • Zygote travels to uterus and implants (week 3-4)
  • HCG hormone starts to be produced (pregnancy test positive)

Common symptoms: Fatigue, mild cramping, breast tenderness, missed period.

Weeks 5-8: Embryo Development

Your baby is now an embryo, and major organs and body systems are starting to form. The heart begins to beat around week 6, and limb buds appear.

  • Heartbeat detectable by ultrasound (week 6-7)
  • Brain and spinal cord begin to form
  • Face features (eyes, nose, mouth) start to develop
  • Baby is about 1 inch long by week 8

Common symptoms: Morning sickness, food aversions, increased urination, mood swings.

Weeks 9-12: End of First Trimester

Your baby is now a fetus, and all major organs are formed (though they'll continue to mature). The risk of miscarriage drops significantly after week 12.

  • Fingers and toes are fully formed
  • Baby can make small movements
  • Genitals start to develop
  • Baby is about 2.5 inches long and weighs 0.5 ounces

Common symptoms: Nausea may ease, breast tenderness continues, bloating, constipation.

Weeks 13-16: Second Trimester Begins

The second trimester is often called the "honeymoon phase" of pregnancy. Many women feel more energetic and experience fewer symptoms.

  • Baby's sex can be determined via ultrasound
  • Baby can hear sounds from outside the womb
  • Hair begins to grow on head and body
  • Baby is about 4.5 inches long and weighs 3.5 ounces

Common symptoms: Increased appetite, back pain, stretch marks, fetal movement (quickening).

Weeks 17-20: Feeling Baby Move

Most first-time moms feel their baby's first movements (quickening) around week 18-20. The baby is active and can kick, stretch, and even suck their thumb.

  • Baby's heartbeat can be heard with a Doppler
  • Vernix (protective coating) covers baby's skin
  • Baby can swallow amniotic fluid
  • Baby is about 6.5 inches long and weighs 10 ounces

Common symptoms: Round ligament pain, heartburn, swollen feet, increased vaginal discharge.

Weeks 21-24: Lung Development

Your baby's lungs are developing surfactant, which helps the air sacs inflate. The baby's skin is still thin and translucent, and they can open and close their eyes.

  • Baby can respond to light and sound
  • Baby has regular sleep/wake cycles
  • Weight gain accelerates (about 1 pound by week 24)
  • Baby is about 12 inches long

Common symptoms: Braxton Hicks contractions, leg cramps, insomnia, hemorrhoids.

Weeks 25-28: Third Trimester Prep

Your baby's brain is developing rapidly, and they can now dream. The baby's immune system is developing, and they can recognize your voice.

  • Baby's eyes can focus on objects
  • Baby weighs about 2.2 pounds by week 28
  • Lungs are maturing but not fully functional
  • Baby is about 14 inches long

Common symptoms: Shortness of breath, frequent urination, swollen hands/feet, back pain.

Weeks 29-32: Baby Grows Rapidly

Your baby is putting on weight quickly, and their movements may feel stronger but less frequent as space becomes limited. The baby's bones are hardening, except for the skull.

  • Baby can turn head side to side
  • Baby weighs about 4 pounds by week 32
  • Baby's skin becomes less wrinkled
  • Baby is about 16 inches long

Common symptoms: Pelvic pressure, difficulty sleeping, heartburn, leaky breasts.

Weeks 33-36: Readying for Birth

Your baby is practicing breathing movements and may drop lower into your pelvis (lightening). The baby's immune system is getting stronger, and they're storing fat for life outside the womb.

  • Baby's head may engage in pelvis (first-time moms)
  • Baby weighs about 5.5 pounds by week 36
  • Baby's organs are fully mature
  • Baby is about 18 inches long

Common symptoms: Increased Braxton Hicks, vaginal pressure, fatigue, nesting instinct.

Weeks 37-40: Full Term & Ready to Deliver

Your baby is full term at 37 weeks and could arrive at any time! Most babies are born between 38-42 weeks. The baby's lungs are fully mature, and they're ready for life outside the womb.

  • Baby weighs 6-9 pounds (average 7.5 pounds)
  • Baby is 19-21 inches long
  • Placenta provides antibodies for first few months of life
  • Baby's head is positioned for birth (vertex position)

Common symptoms: Loss of mucus plug, water breaking, regular contractions, bloody show.