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  • Title: Effects of a behavioural rhythm on conception probability and pregnancy outcome.
    Author: Boklage CE.
    Journal: Hum Reprod; 1996 Oct; 11(10):2276-84. PubMed ID: 9053399.
    Abstract:
    This paper concerns a behaviour-driven weekday rhythm in conception rates in a large natural human population. From 1978 to 1991, North Carolina normal live single births arose from menstrual cycles which began on Monday in clear excess over other weekdays. Cycles beginning on Friday were also in excess. Cycles starting on Saturday and Sunday, or Wednesday and Thursday, each represented significantly less than one in seven of weekly totals. The source of the observed synchrony was a Sunday morning peak of coital frequency. Average cycles which began on Monday had their most fertile day on the most likely day for intercourse, translating a weekly insemination rhythm into a weekly conception rate rhythm. The secondary conception peak in Friday-onset cycles increased with age, to become the major peak for mothers aged > 30 years. We interpret this to represent a previously unreported second type of cycle with a modal follicular phase length of 10 days, the frequency of which increases with age. Several large groups of anomalous human births depart significantly from the weekday rhythm of normal conceptions. These outcomes parallel results of experimental interference with fertilization timing in estruative mammals. We believe that this implicates anomalous fertilization timing in several of the most numerous anomalies of human prenatal development. Time series analyses have identified a weekday rhythm in human conception rates. The present analysis was based on 1.2 million North Carolina birth records for all deliveries beyond 18-20 weeks of gestation in the years 1979-91; 83% included a usable last menstrual period date. In the average week, coital frequency peaks between midnight Saturday and noon on Sunday, resulting in a clear excess of conceptions in cycles that have their 14th day on a Sunday. There was a secondary peak of births from cycles that began on Fridays. Average cycles that began on Monday had their most fertile day on the most likely day for intercourse, translating a weekly insemination rhythm into a weekly conception rate rhythm. The secondary conception peak in Friday-onset cycles increased with age, to become the major peak for mothers over 30 years of age. This finding suggests the presence of a previously unreported second type of cycle with a modal follicular phase length of 10 days, the frequency of which increases with age. Advancing maternal age may increase the frequency of birth anomalies through an increased probability of fertilizations that are poorly timed relative to oocyte maturation.
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