At 34 weeks, a fetus can recognize oft-heard sounds

Reuters - Evidence suggests babies in womb start learning earlier than thought: study

A common way that scientists assess whether a pre-linguistic child has "learned" is how quickly the child gets bored after seeing/experiencing something. If the child is bored quickly, the stimulus is not considered novel by the child, and the conclusion is that the child retains some form of the information (even if she can't use it yet). This was a study of pregnant women who were instructed to read a particular new nursery rhyme out loud 3 times a day for 6 weeks, starting at the 34th week of pregnancy. After repeated readings, the scientists measured the heart beat of the fetus while the mother read the rhyme again. After a few weeks, the fetus' heart rate decreased during the reading, compared to the control group (which had not been regularly read to previously). The decrease in heart rate is believed to reflect a kind of familiarity, or ease, with the rhyme. In contrast, the control (no previous exposure to the rhyme), experienced a slight increase in heart rate, suggesting stimulation of something novel. After a break in reading the rhyme, the mothers again read it aloud and the fetus still had a decrease in heart beat, suggesting retention.