It's good to rely on ovulation calendar but only if ur period regular loh....
Normally alots of gurl misunderstand how to count the "dangerous day" & "safe day".

It's count between the 1st day of period until ur next period. The "dangerous day" will be on the middle 3 days. Coz our ovary will be mature during 3 days(means no matter u wait 35days or 40 days for ur next period to come, it will happend between 3days of the 1st month & second month). If got sperm swim in the ovary, then will get baby loh....

Example: U period on 1st Janauary, and u normally wait 28 days before ur next period. Means the "dangerous day(time when ovary mature)" will fall on 13th-15th January. And sperm can survive in our body for 48hours so start from 11th-15th January, you hav to take action to avoid pregnancy such as use condom. Other than that, the remain days will be "safe day"

I read this from magazine 1......