I'm 20 weeks, so far put on 8kgs. Last pregnancy I put on 20 kgs (i'm 5 foot tall and average size) and then lost 13 kgs in 3 weeks after the birth - baby, placenta, fluid, fluid, fluid! I wouldn't worry provided you're eating healthy options, drinking plenty of water and doing some gentle exercise. The occasional 'naughty' is 'nice' too!