Merp can be used in an catch-phrase to throw people off guard or to have one beg the question Does this person really think he is a grammatitician, I think not! Additionally, one can use the word merp as a euphemism in public as not to offend any people in proximity.
Natural birth to some women means a labour without any medical pain relief, leading to a vaginal birth, and possibly third stage, without any interventions at all. Midwives and obstetricians have their own working definition of natural birth, which sits somewhere in between these different viewpoints.
Natural growth represents the births and deaths in a country's population and does not take into account migration. The overall growth rate takes migration into account. For example, Canada's natural growth rate is 0.3% while its overall growth rate is 0.9%, due to Canada's open immigration policies.