My wife now is in twentieth week of pregnancy. I would like to know which kind of music is the best for the baby's development, of if you can point me to some literature on the subject.
Any music you enjoy listening to, that you plan to play after the baby is born would be appropriate. Human voices are best for learning speech processing.
Newborns can recognize the voices of people whom they heard speaking before birth. Normal sounds, including mother's heart beat, walking, and parents' voices are appropriate for development. Some would say the most important sounds are voices, not music.
The developing brain is processing all the senses in an integrated way. Rhythm-- motion with music-- and prosody (emphasis with speech) are examples of multimodal integration that babies can process before they are born.
Consider taking a multimodal approach to sensory integration, rather than just focusing on one sense organ. Consider reading "The Integrated Development of Sensory Organization" by Robert Lickliter in the December 2011 issue of Clinics in Perinatology https://www.sciencedirect.com/science/journal/00955108/38
Here are some other references.
Based on what I've read (mostly a long time ago), it is good for baby to be hearing classical music in particular throughout gestation and into infant-hood based on anecdotal evidence.
While, admittedly, scientists can't really agree on what is going on, and scientific studies are inconclusive, there is general agreement that classical music has a positive impact of some sort - even just to make baby's neurons fire more often. This TED talk discusses babies learning in the womb in general. Many mothers will attest to babies responding to music before and after birth. There was also a study done that was well publicized around 2001 (the actual study is not available in full online) about babies recognizing music they were played while in the womb even at age one, but the study only tested 11 babies and doesn't seem to have had a control so scientists will generally not site it as scientific (and I have to agree). However, that doesn't mean the anecdotal evidence is completely worthless. This article sums up the commonly agreed upon ideas about music in the womb by the general populace - even if it is still theoretical and anecdotal. It mentions the belief that children will remember what they hear in the womb - based on that same study that only tested 11 children. Keep in mind the article is written by some one who wants to sell prenatal music to expectant parents.
This article speaks more about why the sound does not need to be loud for baby to hear it as well as the difference between what anecdotal evidence has told us about gestational music and scientific studies. Again, the anecdotal evidence seems to be there, but scientific study has been inconclusive.
It is important to be careful about how loud the music is. While sounds are muffled (not unlike what you hear of sounds from the pool deck when you swim), amniotic fluid does conduct sound nicely. Baby can hear you when you are just speaking. I've read one article that says no louder than 70 decibels and another that says no louder than 50. The article I linked agrees with the 50 decibel limit.
I recently read in one article, but also remember discussion about Rap and Rock within the context of the conversation. Many believe these genre's of music aren't generally recommended because some believe that exposure to chaotic and discordant music can have a negative effect. I would argue there are examples of classical music that are a bit "chaotic and discordant" while some rock songs are not at all "chaotic or discordant." Even Beethoven's second symphony has quite a few discordant notes representing the hiccups and burps he frequently struggled with due to gastrointestinal difficulty. Stravinsky is well known for using discordant combinations in his music. When I was pregnant with my little one, "When September Ends" by Greenday was on the radio a lot, so she heard it a lot while I drove to and from work. I swear she started dancing every time she heard it and she still loves the song.
IMHO, Use your judgement. Baby can hear you, music and whatever you are playing and doing through much of the pregnancy - even without any specific effort needed. If you want to do the music during gestation thing, try to choose music that is generally peaceful or up-beat - something that relaxes mom - and just play it on your stereo, but don't get particularly concerned about "perfect choices." There doesn't really seem to be strong evidence one or way or another about the music making a huge difference. Most likely, it impacts the child's ability to relax more than how smart or musically inclined they will be if anything at all.
There doesn't appear to be any evidence to support the idea that there is any concrete long-term benefit for playing music to babies still in the womb. There does appear to be a short-term improvement for specific spatial tasks found after listening to some classical music pieces (the so-called Mozart Effect), but the studies that introduced us to the idea were performed on college students, not infants in the womb.
While I don't think there's any harm to it, you'd probably be better off by having your wife read to the baby, or even just talk, as there are studies that suggest babies can recognize their mother's voice in the womb.
I spent a fair amount of time talking to my wife's belly when she was pregnant, as well. I don't know if it has the same effect as a mother talking, but it certainly made me feel more engaged, and that alone justifies it, in my opinion.
However, it does seem that infants can hear and recognize vocal rhythms and patterns, although the embryonic fluids and the mother's abdomen dampen out all but the lower frequencies. Babies may even be able to differentiate languages:
However, it is important to remember that the benefits of these fascinating aspects of in utero learning are limited:
I've heard all the same suggestions about listening to music and types of music and whatnot, and think it's all a bunch of bunk. Maybe it has some deterministic effect, but even if it does I think it's minor.
The baby's ears are hardly developed, and even if they were they'd be listening through inches of flesh. They aren't going to "hear" much; maybe just the bass rhythm. And there's not really any scientific proof that what one does hear in the womb actually influences development.
Exercise and diet are the things to focus on during pregnancy. I wouldn't worry about much outside that, until science proves otherwise.
After your child is born there are many, many things you can do to influence their development, and at that point music education (among many other things) definitely does have a proven role to play.