Ughhhhhhhhhhhhhhhhhhhhh... I don't know what to say but I really pity you. This is a hands on experience that teaches you the tough lesson that even if you're willing to do the dirty work, it doesn't solve the main problem; her. No matter how much you clean, she is the one not doing any upkeep or preventive measures to keep her house clean.
It doesn't help that there has been no repercussions socially whenever she changes the topic with you and her husband allowing his home to be in this state. Sure she has some health issues but she probably values living in her filth over health. She got accustom to it over time. I've known people who kept eating sugar despite getting diabetes and still did when they were on their death bed. Sadly to say they died from their own denial of reality and valued their desires over their own family and health.
You cannot help, teach, or make her change. She will be defensive about it even if she did want to change, she has to want to get helped herself. She might have to lose everything until she learns her lesson. If you want to do anything to help progress that, it would be an intervention, not temporary help.
She might be more repulsed of changing her comfortable environment due to pregnancy hormones, so expect her to be angry whether or not you do voice your opinions.
One of the things you can offer is that you will be their maid. I find it better that you do get paid because you are wasting valuable time and she is not going to make any effort to keep this home safe. If you hire a cleaner for her, she will be very offended because it's humiliating to her that someone had to intervene and it's someone she doesn't know. She will be upset that she may not have control over making excuses to this stranger or if you had given her a certificate, she'll never use it.
The interventions are usually extreme such as the CPS. Yes the CPS WILL take away her child before deeming that she is even guilty as a precaution to protect the children's health. So that should be a last resort. Once the CPS is in her life, they will never leave her alone. I've had people called the city before that my dad's house was messy when I was little, they didn't do anything, but it gave him a social scare enough to clean up his act. It took a few calls and a brand new home for him to completely stop hoarding.
You can get with the neighbors or mutual friends to help with the intervention personally, taking a photo of their home "before the hoard" and "after the hoard" to let her understand that her home staying clean is possible. She doesn't have to be PERFECTLY clean but at least aim for something in the range of cleanliness between the two pictures. This is not just your opinion, it's her husband, her child, her best friend, her neighbors, her other friends. She will be backed in a corner without feeling hostility as long as you don't raise your voice and you have to sit right next to her. Comfort her that you're here to help and that everyone gets messy sometimes, and now, it's time for some overdue spring cleaning and be able to do small maintenance so she won't feel like she has to ever clean again.
I had MANY animals and used to have more people living with me as well as being pregnant. I still was able to lift up and clean many things. Or at least direct people to help me with cleaning. I'm now 83 pounds, and still have to clean after 7 dogs and 2 cats while holding two kids. I'm diagnosed with anemia from not eating enough so I get dizzy often, yet I could do my and other people's chores. She doesn't have a really good excuse.