I know exactly what you mean - each pg (this is my 3rd) I seem to get to a certain point (around halfway) where I suddenly get terrible constipation, and usually I have just one episode where it just won't come out - I need to go, but..well, it's stuck. It's so bad that you start sweating and feeling like you are going to pass out and it's not fun.
Another thing that may help, if you don't want to be taking Metamucil each day etc, but which is not for everyone, are Glycerol Suppositories. I am yet to use mine, but apparently they are very effective (and not that expensive, about 12 for $5). I bought mine the other day because I'm booked in for a C/S and apparently with a planned C/S they like you to have "gone" within a certain amount of time before you have it. I'd rather be doing this myself at home than at hospital, and thought I'd have some supps handy just in case I can't go due to nerves or something. But like I said, suppositories are not something everyone is willing to use, but hey, when you get desperate, you'll try anything.
Bookmarks