Wayyyyyyy back when we had our first swinging experiences, I stopped them when I wanted to get pregnant (simply because I wanted to be 100% sure the baby was Mr Stuff's). We only came back to the 'scene' after the birth of our last one. I don't see why, barring medical issues, a pregnant woman should stop swinging if she wants to. I think it's more often men that prefer to avoid sexual contact with incubating ladies, most women are very horny during their gestation, and can't get pregnant by anyone else. I can see there might be major problems though, if anything were to go wrong with the pregnancy, whether caused by sexual activity or not. A sense of guilt is a powerful thing and does not distinguish between facts real or imagined.
When Mrs Lost was pregnant she was horny as hell and at times I found it hard to be able to get out of my mind that I was screwing my wife with my child inside her. Whatever the feelings inside me were it remained that there was a child there with us. OK so it had not taken a breath but from the day I saw the scan that child was real and so if at some point that fact occured to me mid fu*k then without anything I could do I would suffer immediate Mr Floppy, not a choice thing.
I think also my feelings at that time were the most priotective and selfish also. It was MY woman and she was carrying MY baby and no other bloke was getting his cock inches away from MY baby, no chance! I could and would provide everything required whether physical or emotional. OK I know it was possesive but I also think that whilst pregnant J was very much "into" the whole two of us+ the baby and a special time to share. I'm afraid the idea to me, and remains so, is that whilst pregnant there is a certain amount of home/nest building and making each other secure with each other as life partners and to allow someone in to be intimate, albeit, in a physical sense with My partner during this time was and would still be incomprehensible.
I would like to say that this is said as a guy who is lucky enough to never experienced marital breakdown or been estranged from their wife or children in any way and can onlyspeak from that point of view.