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I'm a chemistry teacher & was in this position very recently. Definitely look on the CLEAPS cards. They also have a publication about expectant and new mothers that I can't remember the name of but you can find it online & the school schools also provide you with a copy.
You should: 1. Steer clear of all lead compounds. 2. Read up on any transition metal compounds you're using at a level. 3. If other people use your lab or it's near the prep room, ask other members of staff to tell you if they're using high risk compounds or doing risky microbiology or if they have the radioactive sources out. 4. Get someone else (a technician?) to handle radioactive sources for you. 5. Be careful stretching and lifting and don't stand on things. Also if you're moving around site in the snow and ice make a fuss if it's not properly gritted. 6. If in doubt, talk to your union.
Secondly, I teach in FE so at start of course ask the students to let me know (confidentially) if in early stage of pregnancy so I can adjust/arrange for them to be else where. I always refer to CLEAPPS cards for the risk assessment and double check with lab tech. (NB HR and H&S might require separate risk assessments for pregnancy - mine do).
Ones to watch for: depends on level teaching at but most of my organic A2 and transition metal/ligand practicals would be risky off the top of my head. But to be honest GSCE and Access to Higher Education practicals are usually OK as the molar concentrations are low and the risks are more students and glass ware (had two breakages today, one cut finger and tripod burn!)