Progesterone Levels in Early Pregnancy

In the event that the serum progesterone level is 5-25 ng/mL, further testing using US, additional hormonal assays, or serial examinations is warranted to establish the viability of the pregnancy.
from: Pregnancy Diagnosis

Among women who present with a history of infertility, approximately 3-5% of those with infertility have a true luteal phase defect, with repeated cycles lacking adequate luteal support. The incidence of luteinizing hormone deficiency is higher (10-15%) in subsets of patients who are infertile and have had [repeated miscarriages].
From: Luteinizing Hormone Deficiency

...a quantitative serum progesterone level of more than 25 ng per mL (80 nmol per L) proved 99 percent accurate in predicting a living intrauterine pregnancy. Sixty percent of the patients with progesterone values below 25 ng per mL had abnormal pregnancies.
from: Advances in the diagnosis of first-trimester pregnancy problems

Fourteen infertile women and two women with recurrent abortions were suspected of having luteal phase defect (LPD) by BBT charts. In these patients, the midluteal phase serum progesterone (P) levels were determined together with histological examination by the late luteal endometrial biopsies during the same cycle. The diagnostic criteria for LPD was BBT of luteal phase < or = eleven days, mean P level of 3 blood samples taken on three days during midluteal phase was less than 48 nmol/L (15 ng/ml) and the histological dating of endometrium was two or more days behind. The results showed that histological abnormality and low P level were 6.3% and 31.3%, respectively. When histological abnormality combined with lowered P level at the same cycle, the diagnosis of LPD increased to 87.5%. The author suggests that both midluteal phase P level and late luteal endometrial histological examination should be assessed at the same cycle in the diagnosis of LPD. BBT should also be assessed.
from: Diagnosis of basal body temperature, serum progesterone and endometrial biopsy for luteal phase defect