ACCORDING TO MY CALENDAR I HADA PERIOD ON DEC 21, AND O SHLOULD HAD BEEN OVULATING JAN. 4 SO MY PARTNER AND I WERE ACTIVE FROM JAN 1, THROUGH THE JAN 7. I TOOK A PT TEST YESTERDAY THE JAN 18 IT WAS NEGATIVE. COULD A BLOOD TEST DETECT IF I AM PREGANT THIS EARLY. I AM DUE FOR MY PERIOD ON THE 22ND.. PLEASE HELP ME...