Fetal Development: Week by Week

Fetal development is a remarkable process. In just 40 weeks, a single cell grows into a fully formed baby. Here is what happens at each stage, sourced to ACOG, NICHD, and Mayo Clinic.

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Weeks 1-4: Conception and Implantation

Fertilization occurs when a sperm meets an egg, usually in the fallopian tube. The fertilized egg (zygote) divides rapidly as it travels toward the uterus. By week 4, the blastocyst implants in the uterine lining.

At this stage, you may not know you are pregnant. Some women experience light spotting (implantation bleeding), but many have no symptoms.

Weeks 5-8: Embryonic Period

This is the most critical period of development. All major organs and structures begin to form.

Week 5

  • Neural tube forms (becomes brain and spinal cord)
  • Heart begins to beat
  • Embryo is about the size of a sesame seed

Week 6

  • Heart is beating regularly
  • Arm and leg buds appear
  • Eyes and ears begin to form
  • Embryo is about the size of a lentil

Week 7

  • Brain grows rapidly
  • Facial features become more defined
  • Arms and legs grow longer
  • Embryo is about the size of a blueberry

Week 8

  • All major organs have begun to form
  • Fingers and toes are visible
  • Tail disappears
  • Embryo is about the size of a kidney bean

Weeks 9-13: Fetal Period Begins

ACOG states that the embryo is called a fetus from week 11 onward. By the end of the first trimester, all major organs have formed.

Week 9

  • Fetus is about the size of a grape
  • Eyelids form
  • Genitalia begin to differentiate

Week 10

  • Fetus is about the size of a strawberry
  • Fingernails and toenails begin to form
  • Baby can make facial expressions

Week 11

  • Fetus is about the size of a lime
  • All major organs have formed
  • Baby can swallow and urinate
  • Embryo → fetus transition (ACOG)

Week 12

  • Fetus is about the size of a plum
  • Reflexes develop
  • Baby can make a fist

Week 13

  • Fetus is about the size of a lemon
  • Intestines move into the abdomen
  • Vocal cords develop

Weeks 14-27: Second Trimester

The second trimester is a period of rapid growth and maturation. The fetus grows from about 3.5 inches to about 12 inches.

Week 14-16

  • Baby's neck gets longer
  • Baby can make facial expressions
  • Baby's sex may be visible on ultrasound

Week 17-20

  • Quickening (first movement) typically 16-22 weeks
  • Baby can hear sounds
  • Baby's skin is covered with vernix
  • Anatomy scan at 18-22 weeks

Week 21-24

  • Baby's lungs are developing
  • Baby has sleep-wake cycles
  • Baby can respond to sounds
  • Periviable period: 20 0/7 – 25 6/7 weeks

Week 25-27

  • Baby's eyes can open
  • Baby can respond to light
  • Baby's brain is developing rapidly
  • Baby is practicing breathing movements

Weeks 28-40: Third Trimester

The third trimester is a period of rapid weight gain and final maturation. The baby gains about half a pound per week in the final weeks.

Week 28-32

  • Baby is about 15 inches long
  • Baby weighs about 2.5 pounds
  • Baby can open and close eyes
  • Baby's bones are fully formed but soft

Week 33-36

  • Baby is about 17-18.5 inches long
  • Baby weighs about 4-6 pounds
  • Baby's lungs are nearly mature
  • Baby is usually head-down

Week 37-40

  • Baby is full term (37 0/7 weeks)
  • Baby's organs are fully mature
  • Baby is about 18.5-20 inches long
  • Baby weighs about 6-7.5 pounds (average varies widely)

When to Call Your Clinician

Contact your healthcare provider if:

  • You have questions about fetal development
  • You have not felt movement by 24 weeks
  • You have bleeding, pain, or other concerns
  • You are past your due date
Not medical advice: This guide is for informational purposes only. It does not replace professional medical advice, diagnosis, or treatment. Always consult your own clinician.