It's quite common and separation anxiety (with mum or whoever) peaks at around 18 months. It is hard, both my two are clingy with me and sometimes I wish they would just go to dad, but reasure him it does get better. The day I saw my son turn to my DH and ask him for a cuddle just before 2 years of age was such a relief for both of us!!! He still wants mummy most of the time, but if you read 'Raising Boys' which is a great book for you both to read, they really start to hang out with daddy more at about 4 years and less with mummy - it's when they get a huge testosterone boost. Great book