By week 28, the third trimester, studies have shown babies responding to music in-utero.
Of course, the impact of hearing goes much further.
Hearing Starts Forming Early In the Pregnancy
The anatomical development of the auditory system starts in the first 20 weeks of gestation.
However, the neurosensory component, meaning the brain-body connection due to the nervous system, becomes functional around week 25.
From week 25 to 5 – 6 months of age, the neurosensory portion is critical to the continued development of the auditory system.
Outside Stimulation Required
Unlike the visual system, the auditory system requires outside stimulation.
During the second trimester to six months of age, the cochlea, the auditory nerve and the auditory cortex in the brain are tuned to receive signals of specific frequencies and intensities.
This means the baby needs environmental sounds, speech, and music to develop.
Accordingly, controlling outside noise is vital to protecting the sleep-wake cycle of babies in-utero and newborn.
Auditory Learning Can Start As Early As Week 27
A study conducted by Benjamin Arabin, “Music During Pregnancy”, played a soap opera theme song to pregnant mothers.
Other moms-to-be were not exposed to the music. 2 – 4 day old newborns who heard the music in-utero became more alert when the tune was played. 
In a 2013 study, “Prenatal Music Exposure Induces Long-Term Neural Effects”, the nursery song, “Twinkle Twinkle Little Star”, was played five times a week to pregnant women.
The babies were studied at birth and again at four months.
Those who had been exposed to the music reacted to the original tune and to changes to the tune when compared with the control group. 
Language Learning Starts In-Utero
A 2015 study, “Fetal Facial Expression in Response to Intravaginal Music Emission”, showed that fetuses as young as 16 weeks opened and closed their mouths and moved their tongues in response to loud music. Scientists believe this is the first step towards learning a language. 
Cultural cues and intonations also play a role in fetal auditory development. A 2009 study, “Newborns’ Cry Melody is Shaped by their Native Language”, found that French babies cried with a rising melody while German babies cried with a falling melody, parroting the common infections of the two languages. 
Another study conducted in 2013, “Language Experienced In Utero Affects Vowel Perception After Birth: A Two‐Country Study”, included Swedish and American newborns between the ages of 7-75 hours. The babies responded differently to a particular vowel sound in their native and non-native languages. Researchers surmise that babies retain memories of the language they hear in the womb. 
How Prenatal Music Impacts Baby
Research has shown that music aids in gross and fine motor development. Language and body coordination also improves when music is played. 
Does Music Affect Cognition?
In the late 1990’s an industry was born supporting the notion of the “Mozart Effect.” Psychologist, Frances Rauscher, studied American college students and found a small link between IQ and listening to Mozart. Many proponents of this theory emerged supporting the notion that having a baby listen to music in-utero would increase their IQ. 
Another study, “Effects of the Fire Start Method of Prenatal Stimulation on Psychomotor Development: The First Six Months”, points to the fact that music increases alertness.
Some researchers feel this points to higher cognition. However, the reality is that research does not fully support that listening to music will increase a baby’s IQ. 
Strengthening The Bond
The takeaway is that babies will learn to recognize familiar voices before they are even born.
It doesn’t matter whether moms talk to, or sing lullabies to their babies.
By the third trimester, babies know the sound of voices that repeatedly surround them.
Dos And Don’ts Of Playing Music For Moms-To-Be
- Listen to music you enjoy. If the mother is happy and calm, so is the baby.
- Avoid too much noise. Some research studies point to a shortened gestation period, preterm delivery, and decreased birth weight due to exposure to 80-decibel noise levels over a period of time. In fact, living or working near an airport or anywhere with repetitive loud noises can affect some babies.
- This is not the time to experiment. Don’t test out sound frequencies that are out of the ordinary or extreme in volume. Doing so can possibly damage a fetus’ hearing, development, and perhaps the behavioral state.
When Can A Baby Hear Music In The Womb Resources:
 Arabin, Benjamin. “Music during pregnancy.” Ultrasound in Obstetrics and Gynecology 20, no. 5 (2002): 425-430.)
 Partanen, Eino, Teija Kujala, Mari Tervaniemi, and Minna Huotilainen. “Prenatal music exposure induces long-term neural effects.” PLoS One 8, no. 10 (2013): e78946.
 López-Teijón, Marisa, Álex García-Faura, and Alberto Prats-Galino. “Fetal facial expression in response to intravaginal music emission.” Ultrasound 23, no. 4 (2015): 216-223.
 Mampe, Birgit, Angela D. Friederici, Anne Christophe, and Kathleen Wermke. “Newborns’ cry melody is shaped by their native language.” Current Biology 19, no. 23 (2009): 1994-1997
 Moon, Christine, Hugo Lagercrantz, and Patricia K. Kuhl. “Language experienced in utero affects vowel perception after birth: A two‐country study.” Acta Paediatrica 102, no. 2 (2013): 156-160.
 Lafuente, M. J., R. Grifol, J. Segarra, J. Soriano, M. A. Gorba, and A. Montesinos. “Effects of the Firstart method of prenatal stimulation on psychomotor development: The first six months.” Pre-and Perinatal Psychology Journal 11, no. 3 (1997): 151.
 Rauscher, Frances H.; Shaw, Gordon L.; Ky, Catherine N. (1993). “Music and spatial task performance”. Nature. 365(6447): 611. doi:10.1038/365611a0. PMID 8413624
 Lafuente, M. J., R. Grifol, J. Segarra, J. Soriano, M. A. Gorba, and A. Montesinos. “Effects of the Fire Start method of prenatal stimulation on psychomotor development: The first six months.” Pre-and Peri-Natal Psychology Journal 11, no. 3 (1997): 151.
 NCBI: J Obstet Gynecol Neonatal Nurs. 2012 Mar; 41(2): 166–170. doi: 10.1111/j.1552-6909.2012.01342.x