Here's my theory: Since it was a big deal in the comic series, I believe Jenner told Rick that Lori was pregnant. Why? Well, when the survivors entered the CDC, Jenner had them all take a blood test. A blood test can be used to determine pregnancy by showing hCG levels and can detect pregnancy 7-12 days after conception (it's science stuff. I looked it p all by myself!)). It would fit given the amount of time that has passed.
Also, when Rick asked about the blood tests, Jenner replied "Nothing surprising". It would not be odd to Jenner for someone's wife to be pregnant and, since he was planning on killing everyone anyway, there was no reason to let Rick know.
Finally, the look on Rick's face indicates that whatever he was told was a BIG DEAL. The emotion portrayed is about what I'd expect from someone being told their wife is pregnant in the middle of the zombie apocalypse.