Hi Fruitwood,
It's generally accepted that lifting heavy weights when pregnant can put a strain on the stomach muscles and can cause the placenta to detach from the womb if too much strain is placed on the area, which can cause you to bleed a little, and in worst cases miscarry. That said, what defines 'heavy lifting' is very subjective and depends a lot upon your build and fitness level.
Mowing the lawn should be fine, I found that lifting baskets of wet washing placed a strain on me in the first pregnancy and I needed to stop it but this time around I have more strength in my core muscles and can do it with ease. Basically, listen to your body and if something feels like too much effort, don't do it.

You don't need to wrap yourself in cotton wool, keeping up exercise levels is VERY IMPORTANT if you want to keep in shape for carrying the baby through the third trimester, I heartily recommend low-impact exercises like swimming or pilates, which I do myself and plan to continue through the entire pregnancy. I wouldn't recommend a program for weightlifting in the gym, (No Miss Universe for you, this year!) but aerobic exercise is just fine, and if you aren't currently exercising you should probably take up some gentle exercise, believe me, you will be grateful for it in the third trimester when the weight of the baby starts to put a strain on your joints and you need the muscular support.

Do keep in mind that there are a lot of 'shoulds' and shouldn'ts' around, and a good half of them are old wives tales There's a lot of good information around on this site, I hope you enjoy yourself here.

Kerrie