Progesterone in Luteal Phase Deficiency

Inclusion Criteria: * Women who wish to get pregnant with a previous diagnosis of luteal phase deficiency: * Able and willing to sign the Subject Consent Form and adhere to the study visit schedule; * Age: 20-35 years; * BMI: 18-28 kg/m2; * Inadequate luteal phase (menstrual period shorter than 21 days); * Sub-fertile couple: 12 months of trying to conceive without success. * Normal uterine cavity; * Basal P4 level (day 3 of a previous cycle) ≤ 3ng/ml; * Non-smoking; * Fertile male partner (normal sperm count). Exclusion Criteria: * History of recurrent miscarriage; * Basal P4 level (day 3 of a previous cycle) \> 3ng/ml; * Severe uterine malformations (including submucosal fibroids, endometrial polyps, and intrauterine adhesions) ; * Known hypersensitivity to study medication; * Neoplasias (known or suspected breast or genital tract cancer); * Severe impairment of hepatic or renal function; * Use of concomitant medications that might interfere with study evaluations (other hormonal treatment); * Current vaginal infection; * Endometriosis; * PCOS; * Partially or completed block of fallopian tubes; * Hydrosalpinx; * Active arterial or venous thromboembolism or severe thrombophlebitis, or a history of these events; * Porphyria; * A history of idiopathic jaundice, severe pruritus or pemphigoid gestationis during pregnancy; * Antiphospholipid syndrome; * Diabetes mellitus; * Thyroid diseases or autoimmune conditions; * Hypothalamic dysfunction; * Hyperprolactinaemia; * Infertility due to male factor; * Smokers.