Objectives: To define the variability of menstrual cycle length and contribution of follicular and luteal phases to overall cycle variability, and to examine the rise in urinary hCG in early pregnancy.
Methods: Menstrual cycle study. Urine samples from 101 women (recruited from two south-east counties in the UK) were assayed to determine day of luteinising hormone (LH) surge, lengths of follicular and luteal phases and correlations with total menstrual cycle length. HCG study. Daily urine samples collected from 86 women prior to conception until 43 days post-conception were assayed for hCG and examined versus time since LH surge, determined using fertility test kits.
Results: Mean menstrual cycle length was 27.7 +/- 3.4 days, mean follicular phase length was 14.5 +/- 3.4 days and mean luteal phase length was 13.2 +/- 1.9 days. Total cycle lengths varied between and within women. There was a significant correlation (r(2) = 0.70) between follicular phase length and total cycle length; luteal phase length was less variable and showed no association with total cycle length. Concentrations of hCG were significantly similar between women when referenced against the day since LH surge. Three thresholds were determined to indicate time since conception as 1-2 weeks, 2-3 weeks and 3+ weeks.
Conclusions: Total cycle length variation is mainly determined by follicular phase variation and predicting menses onset to estimate time of pregnancy testing is unreliable. Evaluating concentrations of hCG relative to LH surge results in consistent increases between women up to 21 days after conception. Therefore, urinary hCG concentration can be used to accurately estimate time since conception.