Early menstrual cycle impacts of oestrogen and progesterone on the timing of the fertile window.

Study question: What is the effect of oestrogen and progesterone at the beginning of the menstrual cycle in delaying entry into the fertile window?

Summary answer: Both oestrogen and progesterone contribute to a delay in the onset of the fertile window.

What is known already: Oestrogen enhances cervical mucus secretion while progesterone inhibits it.

Study design, size, duration: Observational study. Daily observation of 220 menstrual cycles contributed by 88 women with no known menstrual cycle disorder.

Participants/materials, setting, methods: Women recorded cervical mucus daily and collected first-morning urine samples for analysis of oestrone-3-glucuronide, pregnanediol-3-alpha-glucuronide (PDG), FHS, and LH. They underwent serial ovarian ultrasound examinations. The main outcome measure was the timing within the cycle of the onset of the fertile window, as identified by the appearance of mucus felt or seen at the vulva.

Main results and the role of chance: Low oestrogen secretion and persistent progesterone secretion during the first week of the menstrual cycle both negatively affect mucus secretion. Doubling oestrogen approximately doubled the odds of entering the fertile window (OR: 1.82 95% CI=1.23; 2.69). Increasing PDG from below 1.5 to 4 µg/mg creatinine was associated with a 2-fold decrease in the odds of entering the fertile window (OR: 0.51 95% CI=0.31; 0.82). Prolonged progesterone secretion during the first week of the menstrual cycle was also statistically significantly associated with higher LH secretion. Finally, the later onset of the fertile window was associated with statistically significant persistently elevated LH secretion during the luteal phase of the previous menstrual cycle.

Limitations, reasons for caution: This post hoc study was conducted to assess the potential impact of residual progesterone secretion at the beginning of the menstrual cycle. It was conducted on an existing data set because of the scarcity of data available to answer the question. Analysis with other datasets with similar hormone results would be useful to confirm these findings.

Wider implications of the findings: This study provides evidence for residual progesterone secretion in the early latency phase of some menstrual cycles, which may delay the onset of the fertile window. This progesterone secretion may be supported by subtly increased LH secretion during the few days before and after the onset of menses, which may relate to follicular waves in the luteal phase. Persistent progesterone secretion should be considered in predicting the onset of the fertile window and in assessing ovulatory dysfunction.

Study funding/competing interest(s): The authors declare no conflicts of interest. No funding was provided for this secondary data analysis.

Trial registration number: N/A.