You may work for the employer paying you SMP for up to 10 days without losing SMP. Once you have used your 10 days and you do further work for that employer, you will lose SMP for each week in your Maternity Pay Period in which you do that work. If, after the baby is born, you work for any employer who did not employ you in the 15th week before the week your baby was due, you must tell the employer paying SMP to you about this work and your SMP will stop.
http://www.dwp.gov.uk/advisers/ni17a/