For SMP unless the rules have changed, it basically works out that you have to work there a week before you got pregnant. The guidelines actually state.... "Employed by the same employer without a break for at least 26 weeks into the 15th week before the week your baby is due. Part weeks count as full weeks" and earning 87 quid a week before tax. If not you should still be entitled to Maternity Allowance.
Heres a link to MAternity Allowance explaining that...
http://www.jobcentreplus.gov.uk/JCP/Customers/ WorkingAgeBenefits/Dev_008115.xml.html
If you are pregnant when you get your booking in appointment, all benefit entitlements are explained in the back.