The study aims to evaluate the effect of an increased dose of vaginal progesterone supplementation on the incidence of ongoing pregnancy for patients with a suboptimal serum progesterone level (defined as < 10 mcg/l) on the day of blastocyst transfer.

Inclusion Criteria:

• Informed consent form (ICF) dated and signed
• Age ≥ 18 and < 43 years old
• Body Mass Index (BMI) ≥ 18.5 kg/m2 and < 35 kg/m2
• Less than 4 previous Assisted Reproductive Technologies (ART) cycles
• Current pregnancy wish
• Patients undergoing a single vitrified/warmed single transfer in an artificial prepared endometrium cycle (IVF or ICSI)

Exclusion Criteria:

• Simultaneous participation in another clinical study
• Previous participation in this study
• Known reasons for impaired implantation (specifically: presence of a hydrosalpinx; presence of a type I, II or III fibroid; Asherman's syndrome; uterine malformations, intrauterine adhesions, ≥ grade 3 endometriosis according to the ASRM classification, endometrial tuberculosis)
• Repeated miscarriages (> 2 miscarriages)
• Untreated and uncontrolled thyroid dysfunction
• Tumors of the ovary, breast, uterus, pituitary or hypothalamus
• Abnormal vaginal bleeding without a known/diagnosed cause
• Ovarian cysts or enlarged ovaries
• Fibroid tumors of the uterus incompatible with pregnancy
• Malformations of the reproductive organs incompatible with pregnancy
• Previous antibiotic hypersensitivity reactions (streptomycin and/or neomycin)
• Risk factors for thromboembolic events, such as a personal or family history, severe obesity or thrombophilia
• Active smoking
• Ongoing pregnancy
• Use of carbamazepine, rifampicin or phenytoin
• Those unable to comprehend the investigational nature of the proposed study