Implantation bleeding is light bleeding or spotting that is thought to occur when a fertilized egg (embryo) attaches to the lining of the uterus. There is an indication that around 20-30% of pregnant women experience some form of bleeding in early pregnancy. Of these, 50-60% go on to deliver healthy babies. Because such a large percentage of women who experience early bleeding go on to deliver healthy babies, it is theorized that the cases of harmless bleeding are related to the process of implantation. What makes implantation bleeding a hot topic is that it is often referred to as the first sign of an early pregnancy.

On the research end of the spectrum, there isn't much that is officially known regarding human embryo implantation or related issues like implantation bleeding. Because of this, it is common for any occurrence of early pregnancy bleeding that has no obvious cause to be referred to as implantation bleeding.

Why does implantation bleeding occur?

During the luteal phase of the menstrual cycle, the lining of the uterus typically increases in thickness and blood supply, in preparation for an embryo if fertilization of the egg occurs.

If an embryo is formed, it burrows into this blood-rich lining around 7 to 12 days after ovulation (the same time that fertilization occurs). It is this burrowing of the embryo that is thought to cause a break in the maternal blood vessels, and some blood may escape into the uterine cavity and out through the cervix, ultimately appearing as vaginal spotting. Implantation bleeding is harmless and does not pose a threat to the developing baby.

Signs of Implantation Bleeding

Implantation bleeding usually has the following aspects:

  • Occurs around the time of the expected next menstrual period
  • Is light bleeding and could be just some spotting in most cases
  • Lasts for a short time, generally no longer than two days
  • Is not associated with any significant pain or cramps
  • Does not include any blood clots

When does implantation bleeding occur?

Trying to determine when implantation bleeding will occur can depend on how much information is known about the previous menstrual cycle and when ovulation was most likely to have occurred. Here are a few measurements you can use to benchmark when implantation bleeding might be expected. They all refer to the same timeline, but they use different reference points:

  • Around 10 to 14 days after fertilization/ovulation
  • Around 4 weeks after a last menstrual period (LMP)
  • Around the start of the next expected menstrual period
  • Around cycle days 27-31

What does implantation bleeding look like?

Implantation bleeding can look like a pinkish discoloration of vaginal discharge to bleeding that is similar to a menstrual period. However, it never lasts for longer than two days and there are never any clots in implantation bleeding.

How long does implantation bleeding last?

If it occurs, implantation bleeding lasts for a short time and stops on its own. It could last for a day or two at the most (of fresh blood), and usually no longer. Fresh blood is bright red and fluid in nature, whereas old collected blood is brownish and stringy.

Can implantation bleeding be heavy?

Implantation bleeding is generally described as spotting or light bleeding. Heavier bleeding, which is defined as soaking more than two pads or tampons in an hour, for more than two hours, could possibly be due to other causes of bleeding in pregnancy like a threatened miscarriage or an ectopic pregnancy. These causes need to be ruled out by a healthcare provider as soon as possible if the bleeding experienced is heavy. The need for medical attention is urgent and immediate if the bleeding is accompanied by dizziness, lightheadedness, a racing heartbeat or severe pain.

Implantation bleeding or period?

Because of the time frame where implantation bleeding is experienced and because it occurs before most women can take a pregnancy test, it is possible to mistake it for menstrual bleeding. While the only official way to differentiate between the two is a pregnancy test, there are general differences between what you might expect between implantation bleeding and a normal period.

Implantation bleeding:

  • Last 1 or 2 days
  • Light spotting or bleeding that ranges from pinkish discharge to dark brown spots. Flow does not get heavier as the days pass
  • Cramping is mild to non-existent

Normal period:

  • From 3 to 7 days in length, with 2 to 3 days of bright bleeding
  • Flow of blood starts off heavy and turns lighter
  • Cramping is more severe, typical of what has been previously experienced in a cycle

Is implantation bleeding a sign of early pregnancy?

Possibly. The issue here is that it is difficult to differentiate between implantation bleeding and other more serious types of bleeding in early pregnancy without a clinical examination by medical personnel. Realistically, implantation bleeding can only really be confirmed retroactively.

Various causes for early pregnancy bleeding include:

  • A threatened miscarriage
  • Hormonal deficiencies, e.g. progesterone (a pregnancy support hormone) deficiency. This is a common cause of early pregnancy bleeding that can be treated with hormonal pregnancy support which could help to avoid pregnancy loss.
  • Rarely an ectopic pregnancy. This is an emergency and a medical doctor should be seen immediately.
  • Structural causes of the uterus e.g. uterine abnormalities or a previous cesarean section scar.
  • Bleeding from the outside of the uterus, e.g. from the cervix or vagina. The cervix may bleed more easily during pregnancy because more blood vessels are developing in this area.
  • A molar pregnancy (gestational trophoblastic disease).
  • Certain bleeding disorders.
  • Blood thinning medications.

While some of these causes of early pregnancy bleeding don't occur as early as implantation bleeding would, timing can be an unreliable factor for women who have irregular cycles or are unaware of their date of ovulation.

In general, it is advisable to seek medical attention for all types of bleeding in pregnancy because the range of causes includes harmless implantation bleeding to a life-threatening ectopic pregnancy. It is also possible that a miscarriage due to low progesterone can be prevented. Self-diagnosis of the cause of early pregnancy bleeding should be avoided.