This paper examines whether monetary policy pass-through to mortgage interest rates affects household fertility decisions. Using administrative data on mortgages and births in the UK, our empirical strategy exploits variation in the timing of when families were eligible for a rate adjustment, coupled with the large reductions in the monetary policy rate that occurred during the Great Recession. We estimate that each 1 percentage point drop in the policy rate increased birth rates by 2 percent. In aggregate, this pass-through of accommodative monetary policy to mortgage rates was sufficiently large to outweigh the headwinds of the Great Recession and prevent a “baby bust” in the UK, in contrast to the US. Our results provide new evidence on the nature of monetary policy transmission to households and suggest a new mechanism via which mortgage contract structures can affect both aggregate demand and supply.