Background: Progesterone is produced by the corpus luteum until completion of the luteal-placental shift at approximately 6-10 weeks following last menstruation. Studies have shown that first trimester progesterone levels are predictive of pregnancy viability, and some authors support a level of 5 ng/mL as an absolute threshold to indicate viability.
Case: A 47-year-old woman with recurrent pregnancy loss was noted to have a very low first trimester progesterone level (1.2 ng/mL), but the pregnancy progressed to viability. She unfortunately delivered an intrauterine fetal demise at 27 weeks and 3 days' gestation.
Conclusion: A single serum progesterone level of < 5 ng/mL is suggestive, but not diagnostic, of a nonviable pregnancy. Routine uterine curettage during the evaluation of a pregnancy of unknown location using this level as an absolute cutoff may result in the interruption of a desired, viable pregnancy.