Actually dee, bednobs is wrong, as you need good sleep patterns it would be entirely reasonable to have your risk assessment state that you do not have to work night shifts anymore. Women who work night shifts are more likely to deliver a low birthweight baby, or have longer or shorter pregnancies. This is because you are up at the wrong time and what should be happening in the uterus, might not be happening properly. Presented with this, your employer will have no choice but to either off you days or suspend you on full pay for the rest of your pregnancy. Working between 10 pm and 7pm are the significant times by the way. If you were pregnant and working dayshift, any employer would not be allowed to let you switch to nightshift for these reasons.
In short, tell your employer immediately that you are pregnant and make sure they carry out a risk assement as soon as possible.
Of course some women will deliver very healthy babies after working months of night shifts but we all want the best start for our children and the law is geared to protect you and your baby. Don't feel bullied by it, heavy lifting was a part of my job which could have been easily delegated to someone else but I felt sniped at and victimised already for being pregnant and found myself at the top of ladders and lifting stuff whilst pregnant.