The menstrual cycle is like clockwork for some women, while for others it’s full of surprises. The body goes through various hormonal changes in each phase: proliferative, ovulatory, and luteal. The length of each phase varies from one body to another and also (believe it or not) from one cycle to the next. In fact, women with very regular cycles might be alarmed when their period comes early. They may not be as surprised when it’s late, since the first suspicion is often pregnancy, but an early period can raise suspicions that something is wrong.
Did you know that not every early period is a sign of a problem? In fact, having your period come early is actually super normal1. To understand this better, let’s look at the cycle step by step:
Phases of the Menstrual Cycle
Phase 1: Hormonal stimulation begins right after the end of menstruation. For some women, and in isolated cycles, it can even happen while the endometrium is still being shed from the uterine cavity (menstruation). The follicles start being stimulated and grow day by day.
Phase 2: When the dominant follicle is mature enough, another hormonal action occurs, causing the woman to start secreting higher amounts of estrogen and an LH surge. This is when ovulation happens and, consequently, the fertile window.
Phase 3: After ovulation, the body starts producing another hormone, progesterone. This hormone is secreted by the corpus luteum, which is why it’s called the luteal phase. This phase is usually fixed for each woman. It can last from 11 to 18 days and varies depending on the body or corpus luteum activity.
Why Does Early Menstruation Happen?
Early menstruation happens when one of these phases is shorter than expected. In regular cycles, the proliferative and luteal phases always have the same duration, which is why the cycle days are consistent. However, due to unexpected hormonal changes, the cycle can become shorter.
It’s likely that this shorter cycle is caused by early ovulation, since the proliferative phase is more sensitive to changes than the others2. Sometimes a cycle is shorter because ovulation happened sooner than usual. This time frame can vary from 3 up to more than 10 days in longer cycles.
Irregular Cycles
For irregular cycles, early menstruation is also due to these same hormones. Sometimes the body ovulates on day 12 of the cycle, sometimes on day 22, sometimes on day 8… The change is always in the first phase or during the fertile window, which usually lasts about 7 days but can be shortened to 4 or 5 days due to increased hormone levels.
Anovulation
There are also cases where early menstruation is caused by anovulation3. Women with regular or irregular menstrual cycles can have cycles without ovulation. These cycles can cause two situations: early menstruation or a late period, which can be mild or extreme. It’s important to remember that when your period is late, the first step is to take a pregnancy test, and if it’s not positive, the best solution is to see a doctor.
Hormonal imbalance in a cycle can also trigger early menstruation. This can happen if the amount of progesterone in the body is insufficient to meet the hormonal needs to keep the endometrium in place. So, menstruation can start even if it’s not time. Breakthrough bleeding can also happen for this reason—either low progesterone or high estrogen—even if the woman is in the luteal phase and has corpus luteum activity.
Medication is also a factor. Some hormonal medications can make your period come early. Use of birth control pills or injectables, stopping this type of medication, irregular use or missed doses can cause premature menstruation. The best way to prevent this is to use any medication correctly and only stop with medical advice.
External Factors
Early menstruation can also result from external factors, that is, unrelated to hormones. Everyday stress, for example, can really contribute, as can any event that heavily impacts a woman’s emotions. Sudden shocks or scares are also common causes of early periods.
Note: It’s also important to mention that a delay or advancement of menstruation by up to 5 days is normal. If it goes beyond that, a medical evaluation is advised.
Readers’ Questions:
My period came before the expected day, could I be pregnant?
No. What suggests a possible pregnancy is a delayed period, not an early one. Early menstruation can happen for many other reasons, but if you’re in doubt, take a pregnancy test to rule out the possibility.
How do I tell the difference between implantation bleeding and an early period?
The bleeding is different. Implantation bleeding is usually a light, watery spotting that disappears quickly, while a period comes in a heavier flow and lasts longer.
What could early menstruation with a lighter flow mean?
If your period came early and was much lighter than usual, even with a different color, it could be implantation bleeding. But if it’s early and bright red, it’s likely just a hormonal imbalance.
I take birth control and my period came before I finished the pack — is that normal?
This kind of bleeding before the usual time is called breakthrough bleeding. In general, this is considered normal, but it can also indicate things like missing a day of the pill, a pill with too little hormone for you, or even fibroids or polyps. That’s why it’s always good to check things out, especially if something else unusual happens in the following months. Stay alert!
Is it normal for menstruation to come earlier when switching birth control?
Yes, hormonal imbalances and cycle changes are very common when switching medications since your body may need time to adjust to a new hormone dose. This early menstruation can also happen as light spotting, often called breakthrough bleeding.
Important Tip: If you’re trying to conceive, we recommend having ovulation tests on hand. They can help pinpoint your most fertile day and boost your chances of getting pregnant!
