I think your best bet is to just purchase the cheapest available ticket for the anticipated flight dates now. The lowest published CO fare on CLE-SAN in March is $263 + taxes, in "T" class. It looks like almost all the discounted fares on this route have a $100 change fee. Once the child is born and you can firm up the dates, you'll have to pay the $100, and if "T" class isn't available, then you'll have to "buy up" to the next lowest available fare. In all likelihood, that will still end up costing less than the $600 H fare, unless the availability is really really tight, and you have no choice but to buy up to the H or Y fare (plus the change fee). But you should be able to find cheaper seats on SOME itinerary, connecting through IAH or EWR.

BTW, that H fare is more or less unrestricted, meaning you don't have to worry about fare changes once you purchase the ticket. If you need to change the just the date/time, then all there needs to be is "H" availability. The fare rules regarding changes/penalties are confusing, just because. So you'd certainly lock in the $600 fare (barring the lack of H availability), but I think you can still do better!