This is true because conception may have actually occurred. After fertilization, if there is inadequate hormonal support in the form of progesterone, a miscarriage can occur, even before the period is missed. This is known as luteal phase deficiency. The diagnosis may be suspected by measuring the serum level of progesterone immediately after a positive pregnancy. This test is not routinely done for all pregnant women, and interpretation may be difficult because a low level may not lead to miscarriage, and a high level may not exclude miscarriage.