I believe a big part of it depends on how well you have been doing at work. I got pregnant the first time after about 9 mths on the job, but my boss was okay. Thankfully, it was a smooth pregnancy. The second pregnancy, I tested positive about a week into the job! Luckily, the boss was the same and he was most happy for me. After I had the second one, he told me, "just one more to go!" Because he has 3.
A friend of mine travelled a lot for her job, and she had 2 kids while doing all that sky-jetting, leh... Didn't seem to affect her. In fact, she developed a craving for SQ meals! I think it depends on individual and their health, as well as comfort level. What you can do is make sure you do very well while you can... so when you are sick, or gets too big, they can cut you some slack!
If you have to do heavy lifting or carry things, what you should do is to ground yourself - channel the energy to your legs and feet; and not your abdomen. Like when people practise martial arts, or ballet. This is important; especially in later stages of pregnancy.