At the start of the second phase the birth rate and the death rate are still high, but in this phase they will both decline to a lower level. The death rate starts declining earlier and it will decline faster than the birth rate. This will result in a remarkable growth in the population size during the second phase. A important factor for the decline of the death rate is the improvement in the standard of living. Other factors for this decline were improvement in health care and in hygiene.
The decline in the birth rate during the second phase of the demographic transition theory is also a result of the improvements in the standard of living and in health care. The increase in prosperity resulted in a decline of the total fertility rate. Children were no longer needed to work in the industrial life to help their parents raise an income, and because of the improvements in health care, children were no longer needed to take care of their parents when they were old.