Abnormal cycle. Modern urine pregnancy tests sold in drugstores are very accurate when used properly. Most report accuracy as early as the first day after your first missed period. Many things such as weight changes, excessive exercise and stress can affect your cycle. Your gynecologist should be able to evaluate you for conditions that need treatment.
You don't. You don't. That would be a waste of money, time, and energy. Why would you test before the baby is big enough to be detectable naturally? There is no magic or way to speed up the 'growth' of a fetus to beat nature.