Q: Why has my menstrual cycle synced with my best friend?

A: Women's menstrual cycles can sometimes "sync" with each other if they are in close proximity to one another for an extended period of time.  This is known as the McClintock Effect (named after Martha McClintock, who first discovered it).  In the original study conducted on this effect in the 1970s, it was found that female roommates would often experience "menstrual synchrony" after living together for about 3 months.

The process by which this happens is still unclear.  It's known that many other animals do "sync up" in this way and that this mechanism usually involves pheromones.  For example, when female rats ovulate (go into estrus), they release pheromones that jump-start the ovulation of other female rats that are nearby.  This helps these females to compete with each other for mates.  However, this theory has never been directly confirmed for humans.  Also, research on this effect in humans is mixed, with some studies failing to find any evidence of menstrual synchrony at all.  It's likely that if this does occur in humans, it is somewhat rare and easily overpowered by other factors that affect menstruation (diet, exercise, sleep, etc.).

So, it's either the case that you and your best friend have been around each other long enough that your menstrual cycles have synced up OR it's just coincidence that your cycles are synced right now.  At this point, the science is still out on which answer is the most likely.

Q: ​Can I have sex on my period? Is it harder to get pregnant when on my period?

A: There is no reason why a woman cannot have sex while menstruating.  In fact, having sex and orgasming has been shown to help alleviate menstrual cramping.  However, being on your period doesn't guarantee that you cannot get pregnant.  While a woman's chance of getting pregnant while menstruating is lower than during her most fertile time of the month (i.e., ovulation), women can sometimes start their period while there is still a viable egg in the uterus.  If that egg gets fertilized, then the woman's body will immediately stop menstruating and attempt to build back up the uterine lining in order for the egg to implant.  So, even while menstruating, a woman should use birth control when having sex.

Q: ​Is it possible to have sex while on my period?

A: Yes, absolutely.  Some women worry that it may be dangerous for them or think that it's "gross" to have sex while on their period.  But, as long as it's okay with your sex partner, there's nothing unhealthy or wrong about it.  Some women actually experience heightened sexual arousal during menstruation, due to fluctuations in hormones.  So, sex may even be better for some women while they're on their period.  Either way, there's no reason, other than personal preference, to not have sex while menstruating.