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Motion sickness susceptibility fluctuates through the menstrual cycle.

INTRODUCTION
Surveys of transportation by sea, land, and air indicate that women are more susceptible to motion sickness than men, showing higher incidences of vomiting and reporting more symptoms such as nausea. The cause of this greater motion sickness susceptibility in women is unknown. One suggestion is that it is due to the functioning of the female endocrine system, but there are contradictions between existing studies.


HYPOTHESIS
Motion sickness in women varies across the menstrual cycle.


METHODS
There were 12 healthy women, age 24.4 +/- 6.6 yr (mean +/- SD), not using contraceptive medication, who were exposed to a provocative cross-coupled (Coriolis) motion continued to the point of moderate nausea. The motion stimulus was whole body rotation on a turntable, during which subjects executed head tilts of approximately 45 degrees in batches of eight spaced at 30-s intervals during a staircase profile of rotational velocity which commenced from stationary and incremented in steps of 3 degrees x s(-1) every 30 s. Subjects were tested on: A.) day 5 'menstruation'; B.) day 12 'ovulatory'; C.) day 19 'mid-luteal'; and D.) day 26 'pre-menstrual,' according to a design counter-balanced for order.


RESULTS
Mean +/- SD sequences of head movements required to achieve moderate nausea were: A.) 13.7 +/- 2.8; B.) 14.3 +/- 4.5; C.) 15.4 +/- 6.6; and D.) 16.9 +/- 6.1. The difference between A and D was significant (p < 0.05, 2-tailed).


CONCLUSIONS
There was a trend indicating that motion sickness susceptibility was maximal at day 5 'menstruation', decreasing through days 12 and 19 to a minimum at day 26 'pre-menstrual', to a small but significant extent. However, it is unlikely that this can fully account for the greater susceptibility in women since the magnitude of fluctuation in susceptibility across the cycle is only around one-third of the overall difference between male and female susceptibility.