We analyse the effects of motherhood on women's working career using WHIP, a database that records individual work histories together with childbearing events. In this paper, we model working women's labour supply after childbirth for explaining why some women exit the labour market after childbirth and what are the individual characteristics that make exit more likely. For those working mothers remaining in the labour market we try to identify the working conditions that result in a wage penalty with respect to non-working women. It emerges a significant increase in the probability of transition from employment to non-employment for new mothers, mitigated by the availability of part time jobs. It also emerges that conditional average wages of future mothers are significantly higher than that of non mothers before childbearing and it becomes significantly lower afterward, showing no sign of a closing gap after 4 years. This penalty does not emerge for mothers moving to a part time job after childbearing.