Prenatal diagnosis is among the most crucial stages of prenatal care. Prenatal diagnosis involves taking a thorough background of the mother's health beginning at the time she had her first prenatal visit up to the time of delivery to identify any potential problems. Prenatal visits are designed to provide the most optimal care for mother and the child. A positive pregnancy test can alert you to potential problems that could affect your unborn child. A negative test could result in miscarriage and birth defects.