Unborn Fetus Development

Major Milestones after Conception


Day 1:  Fertilization: all human chromosomes are present; unique human life begins.

Week 2: Rapid development occurs.

Week 3:  The heart starts beating. Liver, kidneys and intestines begin to take shape.

Week 4:  Brain and spinal cord are developing.

Week 5:  Facial features are visible: Hands and feet are forming.

Week 6: About 1-inch long. Brain waves are detectable. Mouth and lips are present; fingernails are forming.

Week 7:  Eyelids and toes form; nose distinct. The baby is kicking and swimming.

Week 8:  About 1 1/8 inches long. All of the major organs take their places. Lungs, taste buds and fingerprints have begun to form, and bones begin to replace cartilage. 

Week 9: About 1½ inches long. Nipples and hair follicles form. Arms grow, elbows and fingernails develop. Baby's toes can be seen. All of the essential organs have begun to grow.

Week 10:  About 2½ inches long. The eyelids begin to close. Facial features become more distinct, the outer ears begin to take shape, teeth begin to form. The baby can hiccup, turn her head and frown.  

Week 11:  About 2½ inches long. The baby can “breathe” amniotic fluid, urinate and grasp objects placed in her hand; all organ systems are functioning. The baby has a skeletal structure, nerves and circulation.

Week 12:  About 3 inches long. All parts necessary to experience pain including nerves, spinal cord and thalamus are present. Vocal cords are complete. The baby can suck her thumb.

Week 13: About 4 inches long. The baby is beginning to form urine and discharge it into the amniotic fluid. Tissue that will become bone is also developing around the head and within the arms and legs.

Week 14: Red blood cells are forming in the spleen. The baby's sex, though known from Day 1, is becoming apparent (for girls ovarian follicles begin forming, for boys the prostate appears).

Week 15:  The baby's taste buds are maturing. The skeleton is forming and will become visible on ultrasound in a few weeks. The arms have almost reached the final relative lengths at birth and baby's neck is more defined.

Week 16:  About 5½ inches long. Bone marrow is now beginning to form. The heart is pumping 25 quarts of blood a day. 

Week 17:  The baby can dream (REM sleep). Fat stores begin to develop under the skin; this fat will provide energy and help keep the baby warm after birth.

Week 18: Baby begins hearing sounds like mom’s heartbeat and the blood rushing through the umbilical cord. The baby may now be startled by a loud noise. 

Week 19:  Vernix caseosa begins to cover your baby. It helps protect the delicate skin from abrasions and chapping that can result from exposure to amniotic fluid. 

Week 20:  About 7½ inches long. The baby is past the halfway point and recognizes mom's voice. The mother might have begun feeling the baby's movements.

Week 21: The baby is becoming more active and can swallow.

Week 22:  The baby discovers the sense of touch. 

Week 23: Fingerprints and footprints are forming. For boys, the testes are descending from the abdomen. For girls, the uterus and ovaries are in place — complete with a lifetime supply of eggs.

Week 24: The baby is regularly sleeping, waking and real hair is growing on her head.

Week 25: Reflexes are developing. The baby begins to respond to familiar sounds, such as mom's voice, with movement.

Week 26: About 9 inches long. The lungs are beginning to produce surfactant which allows the air sacs in the lungs to inflate and deflate. The baby has fingernails.

***All measurements are from the crown of the baby’s head to the bottom of the rump.