Pregnancy: Effects of a behavioural rhythm on conception probability and pregnancy outcome

This paper concerns a behaviour-driven weekday rhythm in conceptionrates in a large natural human population. From 1978 to 1991,North Carolina normal live single births arose from menstrualcycles which began on Monday in clear excess over other weekdays.Cycles beginning on Friday were also in excess. Cycles startingon Saturday and Sunday, or Wednesday and Thursday, each representedsignificantly less than one in seven of weekly totals. The sourceof the observed synchrony was a Sunday morning peak of coitalfrequency. Average cycles which began on Monday had their mostfertile day on the most likely day for intercourse, translatinga weekly insemination rhythm into a weekly conception rate rhythm.The secondary conception peak in Friday-onset cycles increasedwith age, to become the major peak for mothers aged >30 years.We interpret this to represent a previously unreported secondtype of cycle with a modal follicular phase length of 10 days,the frequency of which increases with age. Several large groupsof anomalous human births depart significantly from the weekdayrhythm of normal conceptions. These outcomes parallel resultsof experimental interference with fertilization timing in estruativemammals. We believe that this implicates anomalous fertilizationtiming in several of the most numerous anomalies of human prenataldevelopment.