There are a few possible explanations, and they depend on what your baseline looks like — and if you’ve made any major changes recently, such as going on hormonal birth control or beginning to train for a marathon. "[People] tend to worry when their period isn't exactly the same as usual," Raquel B. Dardik, MD, an OB/GYN at NYU Langone Medical Center, previously told Refinery29. "But there's a large variety of what's normal."
Many people notice that the color and consistency of their period blood changes from day to day. That said, if yours is suddenly light and watery throughout menstruation, and it never was before, it's a good idea to check in with your OB/GYN to see what’s up. Here are some possible explanations.
It’s the first (or last) day of your period.
Some people notice that the first or last day of their period are lighter, with thinner, more watery blood. As Healthline puts it, at the beginning of your period this is “new blood flowing quickly from the uterus." At the end, it’s the result of a low, light flow.
You recently began hormonal birth control
Generally, people who use hormonal contraceptives have less heavy periods, so if you recently started taking the pill, for example, you may notice a change. As Bedsider, an online birth control support network, explains, “The hormonal IUD, the implant, the shot, the pill, the patch, and the ring will all make periods lighter. This is because these methods... make the lining of the uterus thinner. There’s less tissue in the uterus to shed.”
Your estrogen levels are low
Low estrogen can lead to a lighter period that may also appear pink and watery, according to a Greatist article medically reviewed by Valinda Riggins Nwadike, MD. This may be accompanied by other symptoms, including lack of vaginal lubrication, fatigue, headaches, and mood swings. There are a number of possible explanations for low levels of the hormone, including excessive exercising, eating disorders, and health conditions including hypopituitarism (an under-functioning pituitary gland), autoimmune conditions, Turner syndrome, and chronic kidney disease.
You’re close to menopause
Another common explanation for a light, watery flow is perimenopause — the stage when your body is transitioning to menopause. This usually begins sometime in your 40s, though it can be earlier. According to the Mayo Clinic, during this time, your estrogen levels rise and fall unevenly, which can mean major changes for your menstrual cycle.
You have another health condition
While there are many possible explanations for thin menstrual blood, if you’re at all concerned or curious, it’s always a good idea to visit a doctor, who can better evaluate your symptoms.