My HormonologyAccording to a study in the journal Archives of Sexual Behavior, you may worry more that your partner is developing feelings for someone else as you approach ovulation at the end of your Week 2 (around the midpoint of your cycle). As the researchers explain, it’s because high estrogen is pumping up your urge to protect your relationship in case you get pregnant during this phase.

But, you’re not the only one who’s guarding your relationship more closely: Turns out, your mate is also prone to feeling jealous and worried you could stray the closer you get to ovulation, reports the journal Hormones and Behavior. Of course, there could be good reason for your sweetheart to fret: Research shows that when ovulation rolls around, you’re more likely to fantasize about someone other than your current mate and flirt with someone new!