Fertility basics

Implantation Bleeding: An Idea Without Scientific Evidence

Essential Takeaways

  • While implantation bleeding is a popular concept, there is no evidence that it truly exists
  • The physiological process of implantation does not cause bleeding
  • Studies show that bleeding is more likely to indicate an impending period than an early pregnancy

If you notice light blood or brown discharge in the days before your period is due, you may wonder if it’s implantation bleeding. This light spotting is considered by many to be a sign that implantation has occurred and an early symptom of pregnancy. It occurs during the implantation window (6 – 12 days after ovulation), and is said to be caused by the egg burrowing into the uterine wall.

Light bleeding or spotting is common during the premenstrual phase (also called the luteal phase) of a woman’s cycle, whether or not she is pregnant. So how can you tell the difference between normal luteal phase spotting and a true implantation symptom? The answer is that you can’t, and what’s more, there is no scientific evidence that the process of implantation causes bleeding.

How can you tell if it’s implantation bleeding or your period?

You can’t. Bleeding is common in the days before your period is due. It’s also common during the very early stages of pregnancy. When a woman who has premenstrual bleeding goes on to find out she’s pregnant, we call it implantation bleeding. If she’s not pregnant, we simply call it spotting.

This is Monday morning quarterbacking: we decide what to call the bleeding based on whether a woman ends up being pregnant, not based on real evidence that the bleeding was caused by implantation. What’s more, there is no scientific reason to believe that the physiological process of implantation would lead to any bleeding.

Bleeding occurs more frequently in cycles when a woman is not pregnant. Most often, this bleeding is caused by declining levels of progesterone. Progesterone is the hormone that maintains the uterine lining. A drop in progesterone is what causes your period to begin. Progesterone levels can begin to fall a few days before your period is due, and this can lead to light bleeding or spotting before your period.

In cycles where a woman does become pregnant, progesterone levels do not fall, but remain elevated throughout the entire pregnancy. After implantation occurs, the embryo begins to secrete the hormone hCG, which ensures that progesterone does not drop as it does during the menstrual cycle.

Bleeding or spotting during early pregnancy can occur for other reasons, including irritation to the cervix, subchorionic hematoma, infection, or miscarriage. But there is no support for the hypothesis that implantation can produce vaginal bleeding.

What causes implantation bleeding?

It’s common for bleeding to occur during very early pregnancy, but as described above, there is no evidence that this bleeding is caused by implantation. But something must have caused it, so what was it?

By itself, spotting during the luteal phase doesn’t have any conclusive meaning. It can happen when progesterone levels are too low, but it can also happen when progesterone levels are perfectly normal. It can happen when you are pregnant, and it can also happen when you’re not.

Here are several possible alternate explanations for implantation bleeding:

  • Luteal phase bleeding: Some women experience occasional spotting during the luteal, or premenstrual, phase. This spotting may occur regularly with every cycle, or only rarely. It can be caused by declining levels of progesterone in the days before your period is due. When it occurs earlier in the cycle—around a week before your period is due—it may be caused by a brief surge in estrogen levels that normally occurs during the middle of the luteal phase. This surge in estrogen causes a brief dip in progesterone, which can lead to bleeding.
  • Early pregnancy bleeding: While there is no evidence that the process of implantation causes bleeding, bleeding often occurs in very early pregnancy, before or after a missed period, for a variety of reasons. One study found that early pregnancy spotting occurred for nine percent of women, around the time of their expected period. This spotting was usually not heavy enough to be confused with a period. It also tended not to occur on the day of implantation.
  • Chemical pregnancy: Bleeding or spotting can be a sign of miscarriage. For very early miscarriage—called chemical pregnancy—women may notice bleeding around the time of a missed period.

How long does implantation bleeding last?

If you research accounts of implantation bleeding, you’ll find that many women report that it is light and short, and lasts only a few days. It occurs around the time of a missed period, or about 10 – 14 days after ovulation.

Of course, you now understand that the bleeding commonly identified as implantation bleeding most likely is caused by something else entirely.

View sources

Vaginal bleeding in very early pregnancy E.W. Harville, A.J. Wilcox, D.D. Baird, C.R. Weinberg. Human Reproduction, Volume 18, Issue 9, September 2003

Lindsay Meisel

Lindsay Meisel is the Head of Content at Ava. She has over a decade of experience writing about science, technology, and health, with a focus on women's health and the menstrual cycle. Her work has been featured on The Fertility Hour, The Birth Hour, The Breakthrough Journal, and The Rumpus.

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