My GP gave me a form to fill in and send off at 10wks. I received a maternity exemption card soon after and showed that just the once to both pharmacist and dentist. I've found this info for you and hope it helps:
Maternity Exemption Certificates- Frequently Asked Questions
Q. When can I apply for a Maternity Exemption Certificate?
A. You can apply once your pregnancy has been certified by a doctor or midwife, or any time during the twelve months following the birth of your baby.
Q. How do I apply?
A. You need to apply on form FW8 which will be available form your doctor or midwife.
Q. How long will the certificate last?
A. Certificates start from the date one month before the date on which we get your application, to twelve months after the expected date of the baby's birth. If your baby is born later than expected you may need to apply for an extension.
Q. How long will it take for my certificate to arrive?
A. Our Maternity Issue Office will deal with your application. You should expect to receive your certificate within 10 working days of us receiving your application.
Q. I paid for some NHS prescriptions before I realised that I was pregnant, is it possible to claim the money back?
A. You can only claim money back for any charges that you pay on or after the date on which your doctor or midwife signs the application form FW8 to certify your pregnancy. It is not possible to re-claim any NHS prescription charges paid prior to this date.
Q. I am sending off my application form for a Maternity Exemption Certificate today, but I need some prescriptions, will I have to pay?
A. You will need to pay for the prescriptions, but ask for the official NHS receipt, form FP57 (WP57 in Wales). The receipt tells you how to claim the money back. You cannot claim a refund unless you obtain the official NHS receipt.
Q. I've got my certificate but I've paid for some prescriptions between sending the application and getting the certificate. Can I claim a refund?
A. You can claim a refund providing that you obtained the official NHS receipt, form FP57, from you pharmacist. The information about how to claim is shown on the reverse of the receipt. Only in exceptional circumstances can a refund be given without the official NHS receipt.