This is a difficult question to answer because of the lack of really good records of births and deaths from this period and those periods before it. However, the most common explanation is that the growth in population was due mainly to a decrease in the death rate.
The death rate during this century is believed to have decreased as the result of a decline in the frequency of epidemics and famines. These declines can be attributed to improvements in medical techniques and understanding and to improvements in agriculture that were increasing the productivity of land under cultivation.
Because of these changes, it is generally believed that the death rate declined, leading to a growth of population in Europe.