Week 17

Thin skin due to a small amount of subcutaneous fat
Visible blood vessels of scalp are visible and
Development of rapid eye movement

Week 18

Greasy skin due to presence of a cheeselike material known as vernix caseosa. This material consists of dead keratinocytes and a fatty secretion from the sebaceous glands. It protects the skin from the effect of amniotic fluid and from abrasions, chapping, and hardening

Week 20

Visible eyebrows
Visible hair
Body is usually covered with fine downy hair called lanugo which holds the vernix caseosa in place

Week 21

Substantial weight gain
Development of a better proportion of body parts
Wrinkled, pink to red, translucent skin allowing the capillaries to be visible especially in the early part of week 21
Slowing down of the growth
Development of the lower limbs to their final size and proprotions
Development of fetal movement felt by the mother, the so-called quickening. Such movements are felt by the mother around day 147±14

Week 24

Secretion of surfactant by type II pneumocytes of the lung. This secretory product is a surface-active lipid that insures the patency of the alveoli. Despite this ability, a 22-25th week old fetus will die from immature lung development.
Presence of fingernails