If you were strength training before you became pregnant, you will likely be able to stick with the same weights during your first and second trimesters. (For more on first trimester training go here.) I've even had clients progress their resistance during pregnancy, and achieve musculoskeletal strength and endurance gains.
Why would I encourage my pregnant clients to slowly progress their fitness while pregnant? Because we need to be strong as MOMS.
I'll tell you from my own experiences, strollers and carseats and cribs are fucking heavy. You can't tell me that I'm not allowed to lift more than 5lbs overhead when my child's stroller weighs 35lbs.
Now, you do need to be able to lift weights SAFELY. This is the biggest issue I see here. If you were not resistance training before you got pregnant PLEASE HIRE A PRENATAL FITNESS PROFESSIONAL. A "regular" trainer will not know the specific changes that occur during pregnancy that you need to be aware of to train safe. (And no, "being pregnant" or "knowing a pregnant person" does not qualify one as a Pregnancy Exercise Specialist. You need to ask for their certifications and specialty trainings. Mine is with the American Congress of Gynecologists and Obstitricians, and even with that I still need to be on top of the latest Pelvic Floor research, Diastasis Recti Prevention, Pilates/ Breath technique, etc in order to honestly consider myself a specialist. A week or two of education is not enough, in my opinion. *Rant over*)
So, where do you go from here?
Well, stick with the guidelines that apply to cardiovascular fitness during pregnancy. If you can talk and breathe deeply (no "sucking in" or "grunting") during exercise, you're likely okay. If you can think of your fitness on a 1-10 scale, you won't want to be above a 6 or 7. Meaning, if you're huffing and puffing and unable to talk during resistance training (I recommend my clients hum along to the music during group fitness classes, if that's your jam), time to take a break or modify.
If any movement starts to "pull", "bunch", or "strain" your core, swap it for something else. For example, I'm a huge push-up fan, but as my belly grew during my third trimester, I switched to a seated cable fly.
So yes ladies, grab the weights that challenge you, change you, motivate you, and progress you. We need strong legs and glutes for L&D, and strong backs and arms for childcare!