Implantation bleeding typically occurs between six and 12 days after conception, when the fertilized egg attaches to the lining of your uterus. Some women mistake it for their regular period because it can look similar and come around at the time you’d expect your normal cycle.

How can you tell if what you’re experiencing is implantation bleeding? And when is bleeding something to be concerned about?

How Common Is It?

According to Dr. Sherry Ross, OB/GYN at Providence Saint John’s Health Center in Santa Monica, California, implantation bleeding is fairly common and occurs in about 30 percent of pregnancies. In many cases, it is the first sign of pregnancy.

Dr. Linda Burke-Galloway, M.D., M.S., F.A.C.O.G., and author of “The Smart Mother's Guide to a Better Pregnancy,” says, “most women think they’re having a short period that month, when in fact, it’s implantation bleeding. Many women don’t even realize that they are pregnant until they take a pregnancy test.”

How Long Does It Last?

Unlike a regular period, Dr. Burke-Galloway says implantation bleeding is very short-lived, usually lasting no more than 24 to 48 hours, which is the amount of time it takes for the fertilized egg to become implanted into the lining of the uterus.

Dr. Ross explains the timeline as follows:

  • Day 1: first day of menstrual period
  • Day 14 to 16: ovulation occurs
  • Day 18 to 20: fertilization occurs
  • Day 24 to 26: implantation happens and implantation bleeding occurs for about 2 to 7 days

What Does It Look Like?

Typical menstrual bleeding usually lasts for three to five days, starting heavier and then lightening up. Blood from implantation bleeding is typically dark brown or black, which means it’s older blood, although sometimes it can be pink or red as well. It’s also not a heavy flow. You may notice some light spotting of a few drops to slightly larger amounts.

It can be difficult for women to know the difference between implantation bleeding and a regular period, says Dr. Ross, since 50 percent of pregnancies are unplanned, and symptoms can be similar enough to be mistaken.

Here are some major differences.

Regular period:

  • lasts 3 to 7 days, with 2 to 3 days of bright red blood
  • bleeding starts out heavy and lightens up toward the end
  • more severe uterine cramping, which can happen before bleeding and continue for 2 to 3 days

Implantation bleeding:

  • doesn’t usually last for more than 24 to 48 hours
  • bleeding tends to be very light and usually brown, pinkish, or black
  • much more mild (or nonexistent) uterine cramping

When Should You Worry?

All bleeding during pregnancy is considered abnormal. Doctors take it very seriously and encourage pregnant women to report it. Even though not all bleeding is an emergency or a sign of complications, your doctor will likely want to perform tests, such as a vaginal ultrasound, to figure out the cause.

According to Dr. Burke-Galloway, bright red blood means you have active bleeding, especially if you are passing blood clots and are in pain. This could be a sign of a miscarriage or ectopic pregnancy and requires immediate medical attention.

“If the bleeding is taking place in the middle of the night and seems dangerously persistent or heavy, then call your doctor’s practice to speak to the on-call personnel,” says Dr. Joshua Hurwitz, OB/GYN and reproductive endocrinologist at Reproductive Medicine Associates of Connecticut. “In any urgent situation, you can always go to the emergency room to be evaluated.”

Dr. Ross adds, “Every pregnant woman has a 25 percent chance of having a miscarriage. When the bleeding starts to look like a heavy period with blood clots and severe menstrual-like cramping, then it’s time to be concerned that you are experiencing a miscarriage. If heavy bleeding and cramping is associated with tiredness or dizziness, it’s important to contact your healthcare provider to have a pelvic ultrasound, blood count, and beta HCG (human chorionic gonadotropin) to make the correct diagnosis.”