Dan's joke that no one wants to see him hungry because, "There used to be another kid around here," could be an especially dark meta-reference to the Conner's missing fourth child, Jerry, who seems to have been retconned from the series despite surviving the other retcons that occurred in the revival season of Roseanne.
Women can still get pregnant during perimenopause. Darlene being diagnosed with it doesn't automatically mean she's infertile. Perimenopause can start as early as your late 30s and can last up to 10 years before actual menopause sets in. While it's not easy, pregnancy is still possible.