It’s a story as old as time: a few days after your period began, the bleeding suddenly stops. When you throw caution against the wind, you throw on your cutest underpants – but of course the bleeding comes back, stronger and heavier than before. What gives? It turns out that this stop-and-go stream is not unusual.

To understand why, you must first know what is happening in a regular period. Traditionally, a period will occur every 21 to 35 days and last for three to seven days, Megan Gray, MD, FACOG, a board-certified attorney in Florida, told POPSUGAR. At that time, the uterine lining is shed, resulting in the bleeding you have been expecting from that time of the month. The amount of tissue that builds up during the month can vary, making some periods lighter or heavier than others.

Dr. Gray explained that the small blood vessels that feed the uterine lining are also affected by changes in hormone levels that occur naturally during your cycle. “The degree of bleeding from these vessels is directly correlated with the effect of the hormones,” said Dr. Gray. “The shedding of the lining and the bleeding do not occur uniformly along the inner walls of the uterus, which may account for the variation in bleeding day by day within the time period of three to seven days.”

Even if your flow stops and restarts, it can also be normal as long as it happens within the usual time frame for your period (five days is the sweet spot). However, any stop-and-go bleeding that occurs outside this window is probably not normal, Patricia Lenihan, MD, an ob-gyn in Texas, told POPSUGAR. Dr. Lenihan explained, for example, that you should not bleed for three days, then skip four days and then bleed for another five.

Dr. Lenihan noted, however, that when doctors talk about “normal” periods, they only refer to people who are not on contraception. Because different forms of contraception can have a number of side effects (such as reducing the length of your period), it is more difficult to generalize what is or is not normal. “What changes are normal depends entirely on the type of contraception used, and it is therefore always important to discuss cycle expectations with your doctor,” said Dr. Lenihan.