Baby making in the U.S. is on the decline, and the makers of diapers and bottles are feeling the pinch.

The U.S. birth rate dropped to a historic low last summer, as would-be parents pushed back parenthood. Meantime, the $12.6 billion diaper business dropped to an annualized rate of 0.3 percent from 2012 to 2017.

"This could be parents delaying or that drop could be here to stay," said Jason Dorsey, president of the Center for Generational Kinetics. "We won't know for another five years, but companies can't wait that long to find out."

The slowdown could be attributed to any number of factors: focus on work, recession fears or student debt.

The impact of millennials having children later or, in some cases, not at all, is already having an effect.