Q: Which is a better method of tracking fertility — an ovulation predictor kit (OPK) or basal body temperature (BBT) charting?
A: An ovulation predictor kit changes color before you ovulate, based on a surge in your luteinizing hormone (LH), usually 1-2 days before ovulation. OPKs usually predict when you are about to ovulate. A BBT chart reflects your progesterone level. It's close to zero before you ovulate and goes up dramatically after ovulation. This is reflected by a rise in temperature after ovulation.
Your basal body temperature (BBT) chart can detect ovulation only afterward, because your temperature goes up only after you ovulate, usually 1-2 days later.
You can't use a BBT chart to determine when to have sex (or uterine insemination) that same month, but an ovulation predictor kit can predict ovulation early on. You must be aware that sometimes neither will work: both ovulation predictor kits and basal body temperature/cervical mucus charts can be wrong.