How are human beings, especially the fetus, affected by methylmercury?

Methylmercury, also called quicksilver, is a highly toxic form of mercury. Methylmercury is produced by microbes living in wet environments like bogs, the ocean, and rivers. Animals which live in these environments may ingest methylmercury, and as it is bioaccumulative, this becomes more and more dangerous the higher up on the food chain an organism is. Humans most often ingest methylmercury through seafoods like tuna fish. 
When humans ingest too much methylmercury, they may experience heart attack, tremors, loss of vision or hearing, or other forms of central nervous system damage. In the fetus, methylmercury poisoning can significantly affect development and cause lifelong blindness, deafness, microcephaly, and mental and physical impairments. 
Methylmercury has a half life of about 50 days in the human body, but it is much easier for this substance to accumulate in the small bodies of children and fetuses. For people of all ages, doctors recommend limiting the amount of large fish (tuna, walleye, bass) eaten within a certain time frame. Pregnant women should avoid eating tuna to lower the risk of methylmercury poisoning in the fetus.
https://medlineplus.gov/ency/article/001651.htm