Can a brain process auditory signals at 18 weeks of human development?
According to a concise review by Graven &amp; Brown (2008) the development of the auditory system (Fig. 1) in the human is as follows:


15 weeks gestational age*: structural parts of the cochlea and the middle ear are well formed;
20 weeks: middle ear and cochlea are anatomically functional;
Around 25 - 29 weeks: auditory system as a whole becomes functional, as the spiral ganglion cells in the cochlea connect inner hair cells to the brain stem
and temporal lobe of the cortex. In other words, only at this point, the peripheral end organ becomes connected to the central nervous system. At 25 to
26 weeks' gestation, a loud noise in utero will produce detectable changes in autonomic function.
28 - 30 weeks: neural connections to the temporal lobe of the cortex are functional. The development of tonotopic columns in the auditory cortex commences. They are needed to receive, recognize, and react to language, music, and meaningful environmental sounds.


* Gestational age is a measure of the age of a pregnancy which is taken from the beginning of the woman's last menstrual period (LMP), or the corresponding age of the gestation as estimated by a more accurate method if available. Gestational age can be obtained by adding 14 days to the point of fertilization.


Fig. 1. The adult auditory system. source: Graven &amp; Brown, 2007

Reference
- Graven &amp; Brown, Newborn Infant Nurs Rev (2007); 8(4): 187-93