Evolution has but one real goal: to pass along genes from one generation to the next. Which means that as animals we’re still very hard-wired to send out signals (most of them very subtle and even unconscious) that encourage successful conception, so those genes live on in our little ones. A perhaps surprising amount of research has been done on those subtle cues that encourage pregnancy -- including findings that men are more attracted to how a woman smells during peak fertility, and even that female lap dancers receive much higher tips during their fertile phase.
Some research has suggested that one of the unconscious signs that indicates a woman is fertile is the pitch of her voice – specifically, that when she’s ovulating her voice rises in pitch, and also that women’s voices are rated as more attractive during the time they're most likely to conceive. But a small German study just out in the journal PLoS One, didn’t find voice to be a reliable indicator of a woman’s peak fertility. In the study of 23 women, whose voices were studied across their menstrual cycle, the researchers noted, in fact, that 17 of the 23 women had a lower voice on the estimated day of ovulation.
So what is a reliable indicator of peak fertility? Outside of tests in a doctor's office, you can track ovulation by understanding your menstrual cycle, looking for changes in your cervical mucus, and tracking your basal body temperature. Over-the-counter ovulation predictor kits can be useful too.