My former wife and I hated condoms and didn't want her taking birth control, so we didn't start doing MFM threesomes involving penetration until she was pregnant her 2nd time. We did a few FMF threesomes and oral/foreplay MFM threesomes before her last two pregnancies. We weren't into sharing during her first pregnancy unfortunately.
She was unbelievably horny throughout her pregnancies. She was 5'0", 105 lbs, B-C cup breasts before pregnant, E-F while pregnant. She was a beautiful pregnant, with strong muscular legs and firm ass that never dimpled, with no stretch marks. She really took care of herself.
I will say we made sure we knew our partners sexual history and trusted the guys. So we had a limited number of multiple encounter partners. It was unbelievable. I highly recommend not missing the opportunity of the experience if your partner is pregnant. We planned ahead looking for potential guys before she was pregnant. It is my belief that the higher frequency of sex was a health elixir for mom and babies. After we were finished having kids she had her tubes tied and it was "Kitty bar the doors".
I have a couple of stories I wrote about our pregnant threesomes here on the site.