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This change in response is called "habituation." In a second session, the fetus "remembers" the stimulus and the number of stimuli needed for the fetus to habituate is then much smaller.
Based on their research, the scientists found the presence of fetal short-term memory of 10 minutes at 30 weeks. They determined this because a significantly lower number of stimuli was needed to reach habituation in a second session, which was performed 10 minutes after the first session. They also found that 34-week-old fetuses can store information and retrieve it four weeks later. Fetuses were tested at 30, 32, 34, and 36 weeks, and again at 38 weeks.
The 34- and 36-week-old fetuses habituated much faster than the 38-week-old fetuses that had not been tested before. This implies that these fetuses have a memory of at least 4 weeks—the interval between the test at 34 weeks and that at 38 weeks.