This is another case of DH Diplomacy and its how mixed messaging gets started, when a polite firm no in good time would have worked wonders.
I often see people being advised to let the DH deal with it - when half the time its the DH not having a backbone or thinking more of other people's needs than his own partners that cause the issue in the first place! And then I see people waiting around for the DH to deal with it, whilst the DH is hoping the whole horrid mess will somehow go away.
He's not the one who will be 9 months pregnant and he's been trained to do as he's told by CFs.
So yes, your DH should support you, but its also up to you. There is no set rule saying that you can only communicate with them via him. So text them yourself having give DH strong instructions that you are not moving from this position.
DH diplomacy may work in some situations, but this is a straightforward issue of your health and well-being and of your baby's and you are "allowed" to make your own voice heard, indeed it's good practice for the future - so make sure the "event" is properly cancelled.
if he still doesn't "see" the "issue" ( has he been to any appointments with you?) ask your midwife in front of him what she thinks of the idea at your very next appointment.
As he currently doesn't "get it" be prepared for him to be very very wishy-washy in telling them... or leaving it to the last minute so its harder to change everyone's plans because the CFs have already set them in stone. If that happens it will be up to you to deal with it anyway and you will have a situation where DH is OK with it and he's told them he will be extra helpful on the day so you wont get tired then you really are everyone's bad guy.
Dont let this hang over your for the rest of your pregnancy.