Caitlyn Jenner may have inadvertently sparked more speculation that Kylie Jenner is pregnant ... because she says she just learned one of her kids is expecting.

The California gubernatorial candidate was visiting the The Toy Story in Quincy Thursday when she spilled the beans. Caitlyn was telling the folks there about her 18 grandchildren when she joked she wants 30 ... because it's a nice, round number.

That's when she revealed she just recently found out her 19th grandkid is "in the oven." Caitlyn also revealed she's not very good at math ... because she says that means there's only 10 more to go 'til 30.

Now, to be perfectly clear, this does not mean Kylie's pregnant ... it could be one of Caitlyn's sons who's expecting -- Burt, Brandon or Brody -- or her eldest daughter, Cassandra Marino.

To be fair, it could also be one of the Kardashians -- Kim, Kourtney, Khloe or Rob -- or Kendall Jenner.

As we've reported ... Kourtney says she's not pregnant, but the rumor mill's spinning about Kylie because she's back with Travis Scott and she never showed herself at her birthday party. Plus, she was super secretive about her pregnancy with Stormi ... so fans can't help but speculate.

Here's the official tally of Caitlyn's grandchildren -- 4 from Kim, 3 from Kourtney, 1 each from Rob, Khloe and Kylie, 3 each from Cassandra and Brandon, and 2 from Burt.

The 19th's a mystery for now, though ... stay tuned.