I don't believe for a minute that the JWs are the fastest growing religion. But I don't think you are correct to cite the number of babies born to Catholics as being indicative of the rate of growth of the Catholic church. The rate of growth is not about the number of new members. Its about the number of new members compared to the existing membership numbers.
To illustrate my point. Imagine a country - country A - that has 1,000,000 people and another country - country B - that has 100,000,000 people. In 2015, country A has 1,000 new births, while country B has 10,000 new births. Which country grew faster? It was country A - the one with fewer births. How? Because country A had a growth rate of 1,000 / 1,000,000 = 0.1%. But Country B, while having more births, grew by 10,000 / 100,000,000 = 0.01%.
So saying that the Catholic church had 18,000,000 babies born to Catholic parents does not mean that the Catholic religion is growing at a faster rate than the JW religion.