P-631 Day Length Predicts Ovulatory Shifts in Women’s Reproductive Function
Authors: Pearson J , Shanmugam D , Espinosa M , Van Lamsweerde A , Benhar E , Berglund Scherwitzl E , Scherwitzl R , Hill S.E 
Published: June 2022

Abstract

Study question
Is there a relationship between day length and ovulation rate in women of reproductive age?

Summary answer
Analysis showed that increasing day length is associated with higher ovulation rate in women of reproductive age in both Sweden and the United States (US).

What is known already
Historically, winter months have been characterised by threatening survival conditions, such as reduced food availability and lower temperatures. Day length, or photoperiod, provides a reliable indication of upcoming seasonal changes, which promotes season-specific adaptations to behaviours and reproductive systems in mammals. To avoid the survival threats the winter months present, mammals with long gestation periods typically exhibit increased breeding as photoperiods increase. As humans also possess long gestation periods, it may be that humans similarly adapt to increasing photoperiods by exhibiting seasonal shifts in reproductive functioning.

Study design, size, duration
This retrospective analysis of basal body temperature (BBT) data from the Natural Cycles contraceptive app included approximately 115,000 Natural Cycles users’ data, amounting to approximately 1.3 million cycles from May 2014 - January 2021. BBT was used to confirm ovulation, the rate of which was calculated and compared to the number of hours daylight users experienced based on the geographical location in which they had registered for the app.

Participants/materials, setting, methods
We exclude cycles with missing data for any of the studied variables leaving ∼180k Swedish and ∼150K US cycles. The Meteostat library supplied the temperature on the start date of each user cycle. The Suntime library was used to determine the number of hours of daylight users were exposed to on the start date of each cycle, which used the geographic coordinates of the users’ reported country of residence.

Main results and the role of chance
Menstrual Cycle data from approximately 115 thousand Natural Cycles app users aged between 18-62 from two geographic groups, Sweden and the US. Data from cycles during and immediately following the event of a miscarriage, pregnancy, or instance of taking an emergency contraceptive, as well as the two cycles immediately following a completed pregnancy. Using a generalised logistic mixed-effects model, implementation via GPBoost, user-specific variation in ovulation rate as a function of location-specific day length was assessed while controlling for age, cycle length, body mass index (BMI), daily temperature, and education level. We find that day length is significantly correlated with higher ovulation (p &lt; 0.01) in both the US and Swedish cohort.

Limitations, reasons for caution
Ovulation rate and day length may have emerged as a function a several underlying mechanisms, including the optimisation of conditions for childbirth, the optimisation of conditions for conception and early embryonic development, or a decrease in ovulation during the darker, winter months as a result of investment in immune function.

Wider implications of the findings
Photoperiod may be an important regulator of human reproductive activity, providing some preliminary evidence of seasonal changes to women’s ovulatory functioning. Furthermore, birth rate as a measurement of seasonal reproductive patterns in humans may not be as stable or accurate a measure for contemporary analyses of women’s reproductive functioning.

Trial registration number
000000