Human Embryogenesis, Fertilization And Fetus Development

The fetal stage commences at the beginning of the 9th week. At the start of the fetal stage, the fetus is typically about 30 millimetres (1.2 in) in length from crown to rump, and weighs about 8 grams. The head makes up nearly half of the fetus' size. Breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen. The heart, hands, feet, brain and other organs are present, but are only at the beginning of development and have minimal operation.
Fetuses are not capable of feeling pain at the beginning of the fetal stage, and may not be able to feel pain until the third trimester. At this point in development, uncontrolled movements and twitches occur as muscles, the brain and pathways begin to develop.
• Week 16 to 25
A woman pregnant for the first time (i.e. a primiparous woman) typically feels fetal movements at about 21 weeks, whereas a woman who has already given birth at least two times (i.e. a multiparous woman) will typically feel movements by 20 weeks. By the end of the fifth month, the fetus is about 20 cm (8 inches).