Do you think people can change?

i have been with my partner for 3 year im 21 and he is 28 when i meet him he was with someone else and we ended up sleeping together and started to fall for eachother but i was planning to move away for 6 month for work so nothing else happened. when i came back he finished his ex for me and shortly after we moved in together a year after we got engaged and nw we are having a baby it was planned and im due in 2 week. i dont now if it is my hormones but over the last couple of days i have been thinking what makes me so special for him to change because he has admited cheating on his ex with other people apart from me. they where together 2 years they didnt live together or engaged or anything i do trust him but at the min i just feel fat and unatractive and im not really in the mood for it and i just think if he has done it in the past will he do it again. when i think about it i feel abit silly thinking this because if hes not at work hes with me so he wouldnt really have time to see somone else lol. but do you think peopel can change if they find the right person.