He is credited as the father of Finland's public maternity and child health clinic system established in 1922 (Finnish: neuvola), which focuses on monitoring the health and development of newborn and pre-school children and their families, providing parents with childcare guidance and supporting child-parent interaction.

In 1912, he moved to the Imperial Children's Hospital in Berlin, Germany, where he wrote his thesis about infants' bilirubin metabolism in 1913.

While in Germany, Ylppö concentrated on research about children's pathological anatomy and attracted international recognition.

In his research, he observed that deaths of prematurely born infants are usually due to treatable conditions rather than simple underdevelopment, which motivated advancement of treatment of and science related to prematurely born infants.

Ylppö continued his research, wrote extensively to medical journals about child care.