Infant Mortality Rate or IMR can be defined as the number of infants that die under the age of one year out of a total of one thousand live childbirths. Dividing the total number of infant deaths by the total live births in a year, and then multiplying the resultant figure by one thousand can give us IMR of that particular year. Infants usually die due to birth defects (genetic or acquired), low birth weight, weak immunity, poor hygiene, inadequate health and medical facilities, etc. IMR is, in this way, a reflection of the medical and health support given by the state to its citizens. High infant mortality rates reflect the inefficiency of the state to provide adequate healthcare and medical facilities to its citizens. But in general, IMR is low in the developed countries and high in the developing and under-developed countries. We can say that the healthcare facilities have shown a drastic increase throughout the world in the twentieth century. This includes increase in number of hospitals, medical practitioners, nurses, etc. as well as introduction of superior-quality drugs and surgical tools in the market. There has also been an increased awareness regarding sex education, women health during and after childbirth, childcare, etc. We can say that the quality of life has shown improvement, albeit unequally, in different parts of the world. No matter which area of country you focus on, these things directly affect the infant and maternal mortality rates.
One reason why the infant mortality rate dropped steeply in the twentieth century was due to better nutrition. Market milk and baby formulas became a lot more nutrition rich and were tested a lot more frequently to ensure they were clean. As well as advances in medical technology, nutrition became a very important research topic during that time. Women started to watch their nutrition a lot more closely while pregnant and began eating healthier and taking prenatal vitamins more often as well. After birth, women still continued to eat better and more nutrient rich foods which would create better breast milk for the baby and marketers began to create much healthier formulas.
People became more educated on taking care of babies as well. A lot more people learned to not let newborns sleep on their stomachs, which helped greatly to prevent SIDS. They also watched germs more closely, as Caitlynn Reeves said above. With people watching out for germs more closely they were able to keep their children more out of harms way of getting diseases or becoming sick with illnesses that are very hard for a baby to handle and would cause death.
Before the turn of the century, hand washing was not common practice. Germs had not been discovered as the cause of diseases yet. When it was discovered that washing hands could decrease the incidence of bacterial infection, hospitals became much safer places to have babies.
As more women decided to deliver babies in hospitals instead of at home, there were less birthing problems. This, combined with new medical technologies lead to decreased infant mortality rates.