ยาคุมเม็ดแรกกินยังไง

How long do birth control pills take to work?
- It depends on when you start taking them. You can take your first birth control pill any day of the month, but you may need a backup birth control method (like condoms) for the first 7 days.
Combination Pills (COCs)
-  If you start combination pills within 5 days after your period starts, you’ll be protected from pregnancy right away. For example, if you get your period Monday morning, you can start the pill anytime until Saturday morning and be protected from pregnancy right away.

- If you start combination pills any other time, you’ll be protected from pregnancy after 7 days of taking the pill. Use another method of birth control  like a condom or female condom — if you have vaginal sex during the first week you’re on the pill.
https://www.plannedparenthood.org/learn/birth-control/birth-control-pill/how-effective-is-the-birth-control-pill

อันนี้ภาษาไทย
http://www.oknation.net/blog/print.php?id=363516