January Jones found filming 'Mad Men' tough when she was pregnant.

The 35-year-old actress - who is mother to 18-month-old Xander, but refuses to name the father - had her baby bump disguised in a storyline which saw her character, Betty Francis, put on weight because she was unhappy, and the beauty says each day's shooting was hard.

She explained: ''When we started filming, I was quite pregnant and had to come into work at 3am each day for six or seven hours of prosthetic make-up.''

January added she was ''afraid'' of the response to her changing appearance, but found the reaction by fans was even better than she imagined.

She said: ''I was very afraid of how that first episode would look when it aired, but I loved the audience response to her. When she's bigger, viewers are more sympathetic to her.''

This season sees January's alter ego back to her pre-pregnancy figure and the actress loved slipping back into Betty's glamorous outfits.

She told Grazia magazine: ''Betty's style has evolved, not only with time, but also with her role.

''In the first few years, she was emulating Grace Kelly, whereas now she sees herself as more of a Jackie Kennedy.''