You should most likely start gaining weight by at least the middle to end of the 1st trimester if not before.
I have read that most women gain at least 3-5 lbs in the first trimester. I am pretty sure I gained more though.
Throughout pregnancy I gained more than the average person does - but my baby (11 months now) and I are healthy.
The amount of weight you gain really depends on your body, level of exercise and what and how much you are eating.
As far as when you start to show that also depends on various factors such as if you are a smaller or larger build, your pre-pregnancy weight, how your body holds the baby (more tummy or back), and how big your baby is.
Everyone is different just as every baby is different too! So it really varies from person to person!
