It is hard to know the birth rate of JWs for sure. Unlike other fundamentalist religions, JWs are encouraged not to marry or have children, so theoretically they should have less kids. However, my feeling is it is probably similar to the general population.
Watchtower 2008 Apr 15 p.19 Single and childless for a Noble purpose.
"... numerous couples have decided to remain childless ... to remain free to serve Jehovah ... putting Kingdom interests above some of the privileges that go with marriage ... Jehovah will not forget their work, and the love they show for his name... There will be a great tribulation ... it will be difficult time for us adults and children alike ..."
Watchtower 2000 August 1 p.21 Why Do They Have No Children?
"Although Noah's three sons were all married, none fathered children until after the Deluge. While this does not imply that it is wrong to have children today, many Christian couples decline to have children so as to become more fully involved in the urgent work that Jehovah has given his people to do. Some couples have waited for a time before having children; others have decided to remain childless and consider the possibility of bearing children in Jehovah's righteous new world."
Watchtower 1988 March 1 p. 19 Childbearing Among God's People
"If the expansion of Christianity was to come about mainly by childbearing, Jesus would not have encouraged his disciples to "make room" for singleness "on account of the kingdom of the heavens." (Matthew 19:10-12)
How should Christians view marriage and childbearing today, in this "time of the end"? (Daniel 12:4) It is truer than ever that "the scene of this world is changing," or, as another translation puts it, "the present scheme of things is rapidly passing away."-1 Corinthians 7:31, Phillips. Now, as never before, "the time left is reduced." Yes, only a limited time remains for Jehovah's people to finish the work he has given them to do, namely: "This good news of the kingdom will be preached in all the inhabited earth for a witness to all the nations; and then the end will come." (Matthew 24:14) That work must be accomplished before the end comes. It is, therefore, appropriate for Christians to ask themselves how getting married or, if married, having children will affect their share in that vital work.
During all this pre-Flood period, Noah's sons and their wives had no children."