Blood tests, however, have two advantages. They can detect a pregnancy earlier than a urine test and can measure the concentration of HCG hormone in your blood. This is useful information for your healthcare provider in tracking certain problems of early pregnancy. A pregnancy blood test is needed if a urine test is negative but a woman has signs of pregnancy. These signs can be a missed period, breast tenderness, morning sickness, or fatigue.
Pregnancy tests are most accurate if they are done after a menstrual period is missed.
If an early pregnancy test is negative, wait a week or two. The amount of HCG hormone in your body will increase, if you are pregnant, and make the test results positive. If there is a chance you could be pregnant, take health precautions. Avoid drinking alcohol, smoking cigarettes, or using any drug or medication that could harm the baby. Even before a pregnancy test is positive, a baby's nervous system, heart, and lungs are beginning to develop.