HOW TO COUNT YOUR PREGNANCY WEEKS

just wanted to ask how to count the weeks of your pregnancy? Base on my last period, I am 12 weeks pregnant, but base on the scan, I am already 16 weeks pregnant. My last period was April 19, it is just spotting. My normal last period was March 19 but the fact nothing happened with me and my husband during the month since we are LDR. he arrived in April and first days of second weeks of April, that's the time we make love. So just wondering and a bit worried since as they measure the size of the head of the baby, it's 16 weeks already.. Any advice mummys???