E1G Pattern During Embryo Implantation
Estrone-3-glucuronide (E1G) does not exhibit a characteristic "dip" during embryo implantation; instead, E1G levels continue to rise progressively after ovulation in conception cycles, reflecting ongoing corpus luteum activity and early placental hormone production.
Normal E1G Pattern in Conception Cycles
The physiologic pattern of E1G in cycles resulting in pregnancy follows a predictable trajectory:
E1G rises sharply during the late follicular phase, reaching a peak approximately 4-5 days before the LH surge (Day LH-4.7, range -3 to -6), which serves as the primary marker of impending ovulation 1
After ovulation, E1G levels remain elevated or continue to increase in conception cycles, sustained by corpus luteum function and subsequently augmented by trophoblastic hormone production 2, 3
No characteristic decline or "implantation dip" in E1G has been documented in the scientific literature examining urinary hormone patterns during early pregnancy 2, 4, 5
Distinguishing E1G from Progesterone Metabolites
A critical distinction must be made between estrogen and progesterone patterns:
Pregnanediol-3-glucuronide (PdG), not E1G, shows the dramatic post-ovulatory rise that confirms ovulation, with a 7-fold increase from follicular to luteal phase values 2
E1G provides information about follicular development and ovarian estrogen production, while PdG reflects corpus luteum function and progesterone secretion 3, 5
The ratio of E1G to PdG changes across the cycle, but neither hormone exhibits a consistent dip at the time of implantation (approximately 6-10 days post-ovulation) 2
Clinical Monitoring Implications
For women monitoring fertility or early pregnancy:
E1G peaks occur 3-6 days before the LH surge, making sustained rises in E1G useful for predicting the fertile window, but not for detecting implantation 1
Reliable E1G measurement requires correction for urinary creatinine concentration to reduce day-to-day variation from 40% to 25%, with morning urine specimens providing the most consistent results 2
Home monitoring of E1G using validated devices shows good correlation with laboratory radioimmunoassay (correlation coefficients >0.84 in 80% of cycles), though a systematic bias causes home monitor E1G values to read higher than RIA 4
Common Pitfalls to Avoid
Do not confuse the "implantation dip" phenomenon sometimes described for basal body temperature with E1G patterns—these are entirely different physiologic measurements, and the implantation dip concept lacks robust hormonal evidence 3
Avoid interpreting transient E1G fluctuations as clinically significant without considering the substantial day-to-day coefficient of variation (25-40%) inherent in urinary steroid measurements 2
Do not rely on E1G alone to confirm ovulation—PdG rise following an E1G peak provides the definitive evidence of ovulation, with specific PdG thresholds distinguishing anovulatory cycles (with or without luteinized unruptured follicles) from ovulatory fertile cycles 3