EARLY-PREG: A preconception longitudinal cohort of women seeking pregnancy to investigate maternal-embryonic molecular signalling during the peri-implantation window

The EARLY-PREG study is a preconception longitudinal cohort in Chile that systematically collects maternal biological samples across menstrual cycles to characterize time-dependent molecular signatures of maternal-embryonic signaling during the peri-implantation window by comparing conception and non-conception cycles.

The Gist

Imagine you are trying to bake the perfect cake. You know the ingredients (flour, sugar, eggs), and you know the oven temperature, but you don't quite understand the secret handshake that happens between the batter and the heat right before the cake starts to rise. If that handshake fails, the cake collapses.

For a long time, scientists have been trying to understand this "secret handshake" between a mother's body and a tiny, newly formed embryo. This happens in the very first two weeks after conception, a time so early that it's often called the "ultra-early" stage. Until now, it's been like trying to watch that handshake in the dark.

The EARLY-PREG Study is like turning on a high-powered spotlight to watch this process in real-time. Here is how they did it, explained simply:

1. The "Time-Travel" Detective Work

Most studies look at women who are already pregnant. But by then, the "handshake" has already happened, and the clues are gone.

The EARLY-PREG team decided to start before the pregnancy. They recruited healthy women who were actively trying to get pregnant. They treated every woman like a detective solving their own mystery.

• The "Factual" Case: When a woman does get pregnant, that's the "Factual" timeline.
• The "Counterfactual" Case: If that same woman doesn't get pregnant in a different month, that's the "Counterfactual" timeline.

By comparing the two timelines from the same person, they can see exactly what changes in her body when an embryo is present versus when it isn't. It's like comparing a house with a guest staying in it to the same house when it's empty, to see exactly what the guest changed (the dishes, the temperature, the noise).

2. The "Daily Diary" of the Body

To catch these tiny changes, the researchers asked the women to keep a very detailed, daily diary of their bodies. For up to six months, these women collected samples every single day, including:

• Urine and Saliva: Like checking the weather report for the body.
• Blood: To see the internal chemical signals.
• Cervicovaginal Fluid (CVF): This is the study's "star player." Think of this fluid as the messenger pigeon flying between the mother and the embryo. It carries the secret messages (proteins) that tell the body whether to accept the embryo or reject it.
• Cervical Brushings: A gentle "sweep" to collect cells from the doorway of the womb.

3. The "Biobank" Library

All these samples were frozen and stored in a massive digital library called a Biorepository. Imagine a library where, instead of books, the shelves are filled with thousands of tiny vials containing the "molecular fingerprints" of early pregnancy.

So far, they have collected samples from over 1,000 women, with 223 actually participating in the deep study. They have tracked hundreds of menstrual cycles, including 35 successful full-term pregnancies and 17 pregnancies that ended very early (miscarriages).

4. Why This Matters

Why go through all this trouble?

• The "Silent" Period: Before this study, the first two weeks of pregnancy were a "black box." We knew fertilization happened, and we knew the baby was born nine months later, but the middle part was a mystery.
• Solving the Puzzle: By analyzing the "messenger pigeon" fluids (CVF) with advanced technology, scientists hope to find the specific chemical words the embryo uses to say, "I'm here, please let me stay."
• Helping Families: If we understand this language, we might be able to help couples who struggle to conceive or who suffer from early miscarriages. We might find out why the handshake failed and how to fix it.

The Bottom Line

The EARLY-PREG study is a massive, patient, and incredibly detailed project. It's like filming a movie frame-by-frame of the very first moments of human life. By watching the "conversation" between a mother and her potential baby in real-time, scientists hope to unlock the secrets of why some pregnancies start strong and others don't, paving the way for better care for families everywhere.

Technical Summary

1. Problem Statement

The early stages of human pregnancy, specifically the peri-implantation window (the period between fertilization and implantation), remain poorly understood due to the "silent" nature of this phase and the difficulty in modeling human-specific physiology.

• Knowledge Gap: While animal models and in vitro systems exist, they fail to accurately replicate human decidualization, implantation timing, and trophoblast invasion depth.
• Methodological Limitation: Existing longitudinal proteomic studies typically begin analysis at the 8th week of gestation, missing the critical first two weeks where maternal-embryonic molecular signaling (e.g., via Extracellular Vesicles, Early Pregnancy Factor, Preimplantation Factor) establishes pregnancy viability.
• Need: There is a lack of non-invasive, high-resolution, longitudinal data capturing the dynamic molecular exchange between the embryo and the maternal environment (specifically cervicovaginal fluid) in a natural setting without intervention.

2. Methodology

The study employs a preconception, longitudinal, bidirectional, and counterfactual open cohort design conducted in Concepción, Chile (2017–2024).

Study Design & Population

• Cohort: Healthy women aged 18–40 with regular menstrual cycles (21–35 days) and BMI 18–29. Partners (if applicable) were also screened.
• Groups:
  • Trying to Conceive (TCG): Monitored for up to 6 cycles or until pregnancy.
  • Negative Control Group (NCG): Women not seeking pregnancy (abstinent or sterilized) monitored for a single cycle to establish baseline proteomes.
• Counterfactual Approach: Each participant serves as their own control. The study compares the "factual" state (conception cycle) against the "counterfactual" state (non-conception cycle) within the same individual to minimize inter-individual variability.

Data Collection & Biospecimen Handling

• Ovulation Tracking: Initially tracked via ultrasound, fertility monitors, or LH strips, and retrospectively synchronized using hormonal curves (LH, estradiol, progesterone, beta-hCG) from stored samples to define Day 0 (ovulation) with high precision.
• Sampling Frequency: Intensive daily serial collection during the follicular, periovulatory, and luteal phases.
• Biospecimens Collected:
  • Cervicovaginal Fluid (CVF): Collected daily via menstrual cups (central to the proteomic analysis).
  • Other Fluids: Urine, saliva, and peripheral blood.
  • Tissues: Cervical brushings (Days 12–14 and Day 21 post-ovulation), placenta, and umbilical cord blood (for term pregnancies).
• Storage: Samples processed immediately and stored at -80°C or in liquid nitrogen.

Analytical Strategy

• Proteomics: Utilization of dia-PASEF (data-independent acquisition parallel accumulation–serial fragmentation) mass spectrometry for large-scale CVF proteomics, validated for high reproducibility and low-abundance protein quantification.
• Synchronization: Retrospective alignment of cycles based on hormonal confirmation to create precise time-windows (fertile window, pre-implantation, implantation).

3. Key Contributions

• First-of-its-Kind Design: EARLY-PREG is the first study to utilize a bidirectional preconception counterfactual design to investigate maternal-embryonic signaling in the ultra-early stages of pregnancy (Days 0–14 post-ovulation).
• Comprehensive Biorepository: Established a structured longitudinal biobank containing thousands of samples collected during the critical peri-implantation window, a period previously inaccessible for non-invasive human study.
• Methodological Validation: Validated a specific mass spectrometry workflow (dia-PASEF) for CVF that overcomes previous limitations in reproducibility and missing data, enabling the detection of dynamic proteomic signatures.
• High-Resolution Temporal Data: Provides day-by-day molecular profiles synchronized to ovulation, allowing for the isolation of specific physiological windows (e.g., implantation window: Days 7–10).

4. Results to Date

• Recruitment: 1,183 women contacted; 223 enrolled; 174 completed at least one full cycle.
• Cycles Documented: 292 complete menstrual cycles fully sampled.
  • Conception Cycles: 52 (35 full-term pregnancies, 17 early pregnancy losses).
  • Non-Conception Cycles: 240 (including 31 counterfactual cycles and 209 non-counterfactual).
• Biobank Scale:
  • 6,406 CVF samples.
  • 5,120 urine samples.
  • 1,467 saliva samples.
  • 928 blood samples.
  • 293 cervical brushings.
  • 17 placenta/umbilical cord samples.
• Compliance: High adherence to the protocol, with CVF and urine collection rates exceeding 90% in the critical Days 0–14 window. Blood collection compliance was lower (41.6%) due to the intensive schedule.

5. Significance

• Mechanistic Insight: The study aims to decode the molecular dialogue (proteomic signatures) that determines whether an embryo successfully implants or is lost, potentially identifying biomarkers for implantation failure and early pregnancy loss (EPL).
• Clinical Translation: By characterizing the "ultra-early" proteome, the findings could lead to non-invasive diagnostic tools for fertility assessment and early pregnancy viability, moving beyond current reliance on beta-hCG detection which occurs too late to intervene in early failures.
• Future Directions: The cohort is expanding to include a fourth recruitment wave focusing on immunophenotyping (PBMCs) and multi-omics (metaproteomics, metagenomics) to study the maternal immune response and microbiome during the peri-implantation window.
• Resource for Research: The biorepository serves as a foundational resource for the global scientific community to study human reproduction, offering a unique dataset that bridges the gap between animal models and human clinical reality.