8 to 15 weeks
Artist's depiction of fetus 8 weeks after fertilization. The crown-rump length is 1.25 inches.[7]The fetal stage commences at eight weeks when the fetus is typically about 30 mm (1.2 inches) in length from crown to rump and the head makes up nearly half of the fetus' size.[8]. The fetus cannot feel pain, is not yet sentient[9], and moves involuntarily as tissues, organs and pathways begin to develop.[10][11] The movements include motor patterns, and localized movement of the arms and legs, hiccups, stretches and yawns,[10][12] sideward bendings of the head, and generalized movements that involve the whole body.[10]. These movements are involuntary, and the parts of the fetal brain that control movement will not fully form until late in the second trimester, and the first part of the third trimester.[13]
At this stage, the heart is beating but not functional.[14]The hands, feet, head, brain, and other organs are present, but not yet functional.[15] The breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen.[16]
At nine weeks the fetus' involuntary movements include curling toes to move away from an object,[17] and fingers are structurally able to bend. [18] From weeks 9 to 12, the fetal eyelids close and remain closed for several months, and the appearance of the genitals in males and females becomes more apparent.[6] Tooth buds appear, the limbs are long and thin, and red blood cells are produced in the liver, however the majority of red blood cells will be made later in gestation (at 21 weeks) by bone marrow.[19] A fine hair called lanugo develops on the head. The gastrointestinal tract, still forming, starts to collect sloughed skin and lanugo, as well as hepatic products, forming meconium (stool).[20] Fetal skin is almost transparent. More muscle tissue and bones have developed, and the bones become harder. The first measurable signs of EEG movement occur in the 12th week.[21] By the end of this stage, the fetus has reached about 15 cm (6 inches).