What you can and cannot do in the lab is very specific to the work conducted in the lab. Once you are pregnant a risk assessment should be carried out highlighting the dangers to you and the unborn baby with appropriate measures to limit the risk. For example I was working in the lab with Isoflurane, an anaesthetic gas linked to spontaneous abortion. I also worked clinically with Isoflurane and the only restrictions placed on me was that I could not re-fill the vaporisors myself. I was able to use the gas or administer it as long as the system was ventilated to atmosphere. I was also not allowed to work alone both for oncalls and in the lab as this is considered risky for pregnant women. Prolonged standing is not advisable so I was given a chair lol. Regular drinks/snacks breaks may be necessary as often consumption is not allowed in labs etc. Your working hours may need to be reviewed as antisocial or lengthy hours can be considered a risk.
So as you can see the assessment is specific to your work environment and also your pregnancy-related health issues. It is also revisited as the pregnancy progresses or the environment changes. Other peoples work within the lab needs to be considered so that you do not work alongside another 'risky' experiment!