There are now hundreds of fertility tracking apps out there, but it's hard to know if any of them are actually helping. Thankfully, new research suggests that tracking just one important factor — the first day of your period — in apps like these can make it a lot easier to know when your conception chances are best. The study, published earlier this month in the European Journal of Family Planning and Reproductive Health, looked at data from two previous studies. The first one followed 221 women in North Carolina who were hoping to become pregnant. And the second one, performed by the World Health Organization, included data from 706 women in different areas around the globe. The researchers used this data to test a method of predicting the best time of the month for these women to have sex in order to become pregnant. After analyzing this data and running it through a simulation, the researchers found that they could accurately predict women's most fertile time of the month using just the first date of her period. After keeping track of that for six months, the researchers were able to estimate the probability of becoming pregnant at every day of a participants' cycle. For instance, if a woman had a 28-day cycle for the past six months, the researchers calculated that she would be ovulating on the 15th day, making the 13th day the most likely that she would become pregnant (with a 10.9% chance). An algorithm based on this work, Dynamic Optimal Timing (DOT), is already being used by a fertility app called Dot. (The creators of Dot also funded the current study.) Researchers at Georgetown University are now starting up another study that specifically tracks women using Dot to see how well this plays out IRL. The DOT method is best for women with regular reproductive cycles who don't have any other fertility issues. Still, there are other apps out there that use slightly different algorithms. Some of them, such as Daysy, allow you to keep track of extra factors like your basal temperature, another important fertility marker, which may be more helpful if your periods are irregular.