<P> Most pregnancy tests are not accurate until two weeks after ovulation . Knowing an estimated date of ovulation can prevent a woman from getting false negative results due to testing too early . Also, 18 consecutive days of elevated temperatures means a woman is almost certainly pregnant . </P> <P> Calculating the expected due date for a pregnancy based upon the self - reported last menstrual period is less accurate than calculating it based upon either BBT or ultrasound . </P>

What is a spike in your basal body temperature