Major Milestones

Weeks 9-12

• Length doubles between 9 and 12 weeks
• Upper limbs develop to normal proportions, while lower limbs remain less developed
• Male and female genitalia are recognizable by 12 weeks

Weeks 13-16

• Rapid fetal growth occurs
• Fetus double in size
• Lanugo begins to grow, this is fine hair that grow on the fetal body
• Fingernails are formed
• Kidneys begin to secrete urine
• Fetus begins to swallow amniotic fluid
• Fetus appears human
• Placenta is fully formed

Weeks 17-23

• Fetal growth slows
• Lower limbs are fully formed
• Fetal body covered with lanugo,
• Vernix caseosa, white cheesy substance, covers the body to protect the skin from amniotic fluid
• Brown fat forms, fetuses are born with this type of fat to help keep them warm after delivery.

Weeks 24-27

• Skin growth is rapid, and skin appears red and wrinkled
• The eyes open and eyelashes an eyebrows are formed
• The fetus becomes viable, able to live on own, at 26-27 weeks

Weeks 28-31

• Subcutaneous fat is deposited
• If the fetus is born at this time with immature lungs, respiratory distress syndrome may occur.

Weeks 32-36

• Lanugo has disappeared from the body but remains on the head
• Weight gain is steady
• Fingernails are growing
• The fetus has a good chance of surviving if born

Weeks 37-40

• Subcutaneous fat builds, and fetal contours appear round
• Both testes have descended in the male
• The skull is fully developed

Reference:
Simpson, Kathleen Rice: Creehan, Patricia A. (2008). Perinatal Nursing. Lippincott Williams and Wilkins. Philadelphia, Pennyslvania