I was about 14 weeks pg when I started my lecturing position (university). I wasn't pg when I was offered the post and even though I told HR and occ health before I started, I didn't realise they would not pass that information on to my new department. I did feel awkward having to tell them on the first day, so I would advise telling them asap.
Lecturing is hard when you are pg. In the early weeks its students coming in with coffee (the smell would make me sick) and students obviously won't be aware. Once they know, I found most were lovely and it might be worth sharing the info with students sooner rather than later. Depending on your lecturing style, you can also make a bit of a joke about the effect the pregnancy is having on you, kind of, "wow, this is so amazing it takes my breath away. Oh no, that's just baby kicking", kind of thing. I tend to be very mobile when lecturing - I find it is a necessity if you have 200-300 students in a room, but this can be difficult as you get bigger in later months.
Always have a glass of water with you at the front, so if you get breathless, you can take a drink, which helps a bit. For small group teaching (at university, this usually means 20-30 or less), you might be able to get away with sitting down some of the time. Finally, you might find it helps to make narrated PowerPoints of your lectures available to students in advance, which can allow you to get through material faster and use the timetabled slots to help with specific questions/problems, taking the pressure away from you being at the front 'performing'. My last university was quite keen on using this, students liked it (and no, it didn't result in a mass exodus of students).
Much probably depends whether you are HE or FE though and if this is your first lecturing post. In HE for a first post you will usually get a (very) reduced teaching load anyway.