BACKGROUND The aim of this research is to contribute to knowledge of the normal spontaneous motor behavior of the human fetus during the second trimester of pregnancy. This study focuses on five patterns of spontaneous fetal movement: startle (S), axo-rhizomelic rhythmia (ARR), axial stretching (AS), general movement (GM), and diaphragmatic contraction (DC). METHODS A cohort of 13 subjects was followed up using 2D obstetrical ultrasound images at 12, 16, 20, and 24 weeks of gestation. As inclusion criteria, neonatal neurological examination and general movements after eutocic delivery at term were normal in all of the subjects, and their neuromotor and cognitive development until the end of pre-school age were also normal. RESULTS All these five motor patterns are present at the beginning of the 2(nd) gestational trimester, but their quantitative and qualitative traits are diverse according to gestational ages. The phasic, isolated or rhythmically repeated movements, S and ARR, are prominent at 12 and 16 weeks of gestation, and then their presence gradually diminishes. By contrast, tonic and complex AS and GM movements increase their presence and quality at 20 and 24 weeks. RAR constitute a particular periodic motor pattern not described in previous literature. Moreover, the incidence of DC is progressive throughout the trimester, in clusters of 2-6 arrhythmic and irregular beats. Fetal heart rate increases during fetal motor active periods. CONCLUSIONS All five normal behavioral patterns observed in the ultrasounds reflect the progressive tuning of motor generators in human nervous system during mid-pregnancy.