What is implantation bleeding?

Implantation bleeding is light bleeding that may occur about seven to 14 days after fertilization. It's caused by a fertilized egg implanting in the blood-rich lining of your uterus.

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Implantation bleeding, explained

Implantation bleeding is light bleeding that you might experience if a fertilized egg implants in the lining of your uterus. According to the American College of Obstetricians and GynecologistsOpens a new window (ACOG), this can happen 1 to 2 weeks after fertilization.

Here's why experts think it happens:

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After the sperm fertilizes the egg, it starts the three- or four-day trip from the fallopian tube to your uterus. During this time, it divides into hundreds of identical cells. Meanwhile, your ovaries release estrogen that causes the lining of the uterus (the endometrium) to thicken and develop more blood vessels. The uterine lining is preparing to host the quickly growing embryo.

Once the embryo enters the uterus, it's called a blastocyst, a tiny ball of several hundred cells. A day or two later, it begins burrowing into the blood-rich lining of your uterus, where it continues to grow and divide. As it burrows into the endometrium, the blastocyst may cause little blood vessels to burst, resulting in a small amount of vaginal bleeding.

Implantation bleeding symptoms

In addition to light bleeding, some women also have these other implantation symptoms:

  • Light cramping (implantation cramps)
  • Nausea
  • Breast tenderness
  • Lower back pain
  • Mood swings
  • Headaches

These symptoms are no guarantee that you're pregnant – breast tenderness can be a sign of ovulation or PMS, for example. And several others – cramping, headache, backache, and mood swings – can also be signs of PMS. Taking a home pregnancy test is the best way to find out if you're pregnant. (Though you may have to wait a few more days to get an accurate result.)

If you do get your period instead of a positive pregnancy test, and haven't yet scheduled a preconception visit with a midwife or doctor, now's a good time to do so.

When does implantation bleeding occur?

Implantation bleeding tends to occur about seven to 14 days after conception – about the same time or just before you'd be expecting your period. This is around the same time a fertilized egg would be attaching itself to the uterine lining.

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How long does implantation bleeding last?

Unlike a regular menstrual period, implantation bleeding usually lasts no more than a day or two, three at the most. This is the amount of time it takes for the fertilized egg to become implanted into the lining of the uterus.

How common is implantation bleeding?

Implantation bleeding doesn't happen in every pregnancy. Between 15 and 25 percent of pregnant women experience some light bleeding or spotting in the first trimester. Some of this is due to implantation bleeding.

Some women don't realize they're pregnant and think they're having a light period when they see implantation spotting, especially because it happens around the time you'd expect a period.

How can I tell if it's implantation bleeding or my period?

Here's how to tell whether your bleeding is more likely to be implantation bleeding or your period:

  • Amount of blood. Implantation bleeding is a lot lighter than a typical period. It's usually just a little spotting.
  • Length of time. Most women bleed for three to seven days during their period. Implantation bleeding often lasts only one to three days.
  • Color. Menstrual blood is usually bright red or dark red, but implantation bleeding tends to be a light pink, brown, or rust-colored discharge.
  • Clotting. Many women bleed enough during their period that some of the blood clots or becomes thick like a gel. Implantation bleeding is too light to clot.
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When to see a doctor for bleeding in early pregnancy

If you have bright-red bleeding at any point after a positive pregnancy test, or if you develop other symptoms (such as pelvic or abdominal pain, dizziness, or lightheadedness), call your provider immediately. Bleeding and spotting during pregnancy is common, but even so ACOG recommends contacting your ob-gyn if you have bleeding at any time during pregnancy.

You may need an exam to make sure you don't have an ectopic pregnancy. This happens when the fertilized egg grows outside the uterus, usually in a fallopian tube. (It's possible to have an ectopic pregnancy even if you don't get a positive result on a pregnancy test.)

Bleeding or cramping in early pregnancy may also be a sign of a molar pregnancy (when the cells in the placenta don't develop normally) or an impending miscarriage.

"Many women spot in the first trimester for no apparent reason and go on to have a healthy pregnancy," says Shannon SmithOpens a new window, an ob-gyn at Brigham Faulkner Ob/Gyn Associates in Boston. "But if you're worried for any reason, or if you have other symptoms, don't hesitate to get in touch with your healthcare provider."

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ACOG. 2018 Early pregnancy loss. American College of Obstetricians and Gynecologists. a new window [Accessed April 2023]

ACOG. 2022. Bleeding during pregnancy. American College of Obstetricians and Gynecologists. a new window [Accessed April 2023]

Cleveland Clinic. 2022. Implantation Bleeding. a new window [Accessed April 2023]

March of Dimes. 2020. Bleeding and spotting from the vagina during pregnancy. a new window [Accessed April 2023]

Mayo Clinic. 2022. Is implantation bleeding common in early pregnancy? a new window [Accessed April 2023]

NIH. 2017. What are some common signs of pregnancy? National Institutes of Health, National Institute of Child Health and Human Development. a new window [Accessed April 2023]

UpToDate. 2023. Evaluation and differential diagnosis of vaginal bleeding before 20 weeks of gestation. a new window [Accessed April 2023]

Karen Miles
Karen Miles is a writer and an expert on pregnancy and parenting who has contributed to BabyCenter for more than 20 years. She's passionate about bringing up-to-date, useful information to parents so they can make good decisions for their families. Her favorite gig of all is being "Mama Karen" to four grown children and "Nana" to nine grandkids.