Hi there. My IBS-like symptoms have been brought on by having part of my colon removed (rectal cancer) and/or my gall bladder out, 9 years ago. While I've never been officially diagnosed with IBS-D, my symptoms mimic so many of you here and I've been able to find wonderful support on this forum. So please stick around! Warmest congratulations on your pregnancy!
I'm 53 now so my periods are hit or miss, usually 3 months apart. But when I was regular, I noticed loose bm's, long before my IBS issues started. Someone once told me that our periods are nature's way of cleaning us out (bowels too). I could almost predict when I would start because the loose bm's would come. I noticed this connection as far back as my 20s. Many women here on this site have mentioned IBS flares before their periods start.
What I would suggest is eating foods that firm up your stool. Just tackle this increase in D, with a low residue diet (what I still follow, 9 years now). I would concentrate on foods such as bananas, mashed potatoes, applesauce, white bread/bagels/english muffins, chicken, turkey, fish, plain or vanilla flavored yogurt. I would also avoid foods or beverages that increase D - whole wheat products, fresh produce such as berries, pineapple, baby lettuce greens, hot tea, coffee, caffeine, carbonated beverages, etc. Try this for a 24 to 48 hour period and keep a food journal. See if you notice an improvement. If so, ask your OB if you could go on a low residue diet while pregnant or at least for this transitional stage (your GI tract getting used to hormonal surges).
I hope you feel better.....hang in there. You found the right place to vent, etc.