The EARLY-PREG open cohort: the first preconception and longitudinal cohort designed to characterize proteome and immunological signatures of early embryo-mother crosstalk

Embryo–maternal crosstalk is essential for pregnancy establishment and its success. This exchange of information between mother and embryo begins shortly after fertilization and involves complex biochemical exchanges, yet its dynamics before implantation remain poorly understood in humans. EARLY-PREG is a preconception, longitudinal cohort planned to investigate the molecular signatures of maternal–embryonic communication through systematic collection of maternal fluids and tissues during the first two weeks after ovulation. The study integrates proteomic, immunological, and physiological data to build a multidimensional profile of conception success.

Between 2017 and 2024, three recruitment waves enrolled healthy women seeking pregnancy in Concepcion, Chile. Participants completed detailed health, lifestyle, and sociodemographic questionnaires. Menstrual cycles were prospectively monitored for up to six consecutive cycles or until conception. Each cycle included systematic daily sampling of cervicovaginal fluid (CVF), urine, saliva, and blood. Ovulation and fertile windows were confirmed by ultrasound, fertility monitors, and/or luteinizing hormone (LH) strips. Hormonal curves (LH, oestradiol, progesterone, β-hCG) from stored blood and urine were retrospectively analyzed to refine the timing of biological events.

To date, 1,183 women have been contacted; 223 met eligibility criteria. Of these, 129 women actively trying to conceive completed at least one cycle, resulting in 35 full-term pregnancies and 20 early pregnancy losses. In total, 293 cycles have been fully documented: 55 conception cycles and 238 non-conception cycles, including 33 counterfactual and 205 non-counterfactual cycles. Additionally, 40 abstinent and 5 sterilized women completed the protocol as part of a negative control group. The high-compliance biorepository contains multi-fluid samples from all cycles, enabling detailed characterization of early pregnancy. Latest analyses compare proteomic profiles from conception cycles with matched non-conception cycles to identify protein networks associated with successful implantation. Future Directions: The fourth recruitment wave is planned to begin in 2025, focusing on immunophenotyping maternal peripheral blood mononuclear cells (PBMC) and continuous tracking of vital signs across the menstrual cycle. Vital measures will include basal body temperature, heart rate, and respiratory rate. This expansion aims to integrate immune profiling and physiological monitoring with proteomic data, enabling a more comprehensive understanding of early pregnancy biology.

EARLY-PREG is the first large-scale, preconception open cohort to combine daily multi-fluid sampling with advanced molecular and physiological measurements during early pregnancy. By integrating proteomics, immune profiling, and physiological monitoring, this cohort is poised to enhance our understanding of the earliest determinants of human fertility and to enable the development of predictive and personalized approaches in reproductive health.

Embryo-maternal Crosstalk; preconception open cohort; Early pregnancy; Longitudinal study; Reproductive health