To get SMP you will need to have been there for 26 weeks before the 15th week before your baby is due. As a normal pg is 40 weeks it basically means that you will have to have been there a week before you become pg, so you will not be entitled to SMP (which is normally 90% of your wages for 6 weeks, followed by Maternity Allowance for 20 weeks). You will be entitled to Maternity Allowance which if you have worked for 26 weeks in the 66 weeks up to when the baby is born (which you would have). Maternity Allowance is £106 per week for 26 weeks, it can start up to 11 weeks before your baby is due.
However the company may be generous and offer maternity benefits above what the law states they have to pay. If this is the case and they do pay you anything about MA, then if you don't go back to the company you will have to pay them the difference back, however if all you have received is MA you won't have to pay a penny back as the government is paying you this if you decide not to return.
Hope this makes some sense. I would've changed my job, but I knew we were trying for a baby and I didn't want to lose out on my maternity benefits had I found out I was pg.
Although you don't have to tell your employer until 15 weeks before you are due, in order not to lose out on pay for antenatal appointments they'll need to know (and they might get suspicious if you have a growing tum, lol)
