What Is Implantation Bleeding?

Implantation bleeding is spotting or light bleeding that happens when a fertilized egg attaches itself to the uterine lining (endometrium). Implantation bleeding may occur six to 12 days after fertilization, but not everyone experiences it.

Approximately 15% to 25% of those who become pregnant will have bleeding in the first trimester. Implantation bleeding is a normal symptom that doesn’t require any medical attention.

However, there are important signs to watch for that can indicate something serious.

When Does Implantation Bleeding Occur?

Implantation bleeding occurs at a specific point during the menstrual cycle—typically between days 20 and 24, assuming you have a 28-day cycle.

It only occurs when a fertilized egg attaches itself to the uterine lining. Still, someone can be pregnant and experience no implantation bleeding at all.

Obviously, this is not the only type of bleeding you may experience. Ovulation bleeding is light bleeding or spotting that occurs when an egg is released from the ovary. Period bleeding occurs if a fertilized egg does not implant in the wall of your uterus after ovulation and the lining sheds.

You may assume implantation bleeding is the start of your period since the timing of both are similar.

Your menstrual cycle begins on the first day of your period. If you experience light bleeding 20 to 24 days after the first day of your last period, it might be implantation bleeding. Bleeding that begins 28 days after the first day of your last period is more likely to be period bleeding.

What Implantation Bleeding Looks Like

Implantation bleeding can last from a few hours to a few days. It may come and go or remain constant during that time.

Some symptoms of implantation bleeding include:

• Light cramping
• Headache
• Nausea
• Lower backache
• Mood change
• Breast tenderness

Though implantation bleeding and period bleeding can be confused for one another due to when they occur, there are differences between the two.

Implantation bleeding is one of the earliest signs of pregnancy. Since these two causes of bleeding can be confused, taking a pregnancy test is the best way to know if you are pregnant.

Why Implantation Bleeding Happens

When an egg is released from an ovary, usually once per month, ovulation occurs. At this point, the egg travels through the fallopian tube.

If an egg is penetrated by a sperm and becomes fertilized due to intercourse, it starts to divide into multiple cells while moving through the fallopian tube to the uterus.

A fertilized egg takes several days to travel down the fallopian tube before either implanting or being shed by the body if implantation fails. If it does implant, human chorionic gonadotropin (hCG) hormone is produced in the uterus, preventing the uterine lining from being shed.

If implantation does not occur, the egg and uterine lining are shed during the menstrual period.

After dividing into about 100 cells, the egg becomes what is known as a blastocyst. The uterine lining has many blood vessels, so when the fertilized egg (blastocyst) pushes into the lining, bleeding can occur.

Other Causes of Bleeding

Early miscarriage and ectopic pregnancy are two possible reasons for vaginal bleeding. These are medical emergencies, so you should be aware of the symptoms and seek care right away if you experience them.

That said, there are several other possible causes of bleeding that are not concerning—even expected. Others require non-urgent medical evaluation and possibly treatment.

Some of these possibilities include:

• Beginning, ending, or missing doses of hormonal contraception
• Sexual intercourse
• Fertility treatments and drugs
• Cervical erosion
  
• Endometriosis
  
• Diseases of the thyroid
• Polycystic ovary syndrome (PCOS)
• Fibroids or polyps
• Infections

If bleeding occurs more than a few days after you expected to get your period, it likely is not implantation bleeding.

Cervical Erosion

Cervical erosion (cervical ectropion) is a condition in which the cervical canal turns outward, exposing the cervix’s inner cells to the vaginal canal’s acidic environment. This causes it to become red, raw, and potentially bleed.

This bleeding can happen as a result of the cervix being irritated during a pelvic exam. Sometimes this happens after sexual intercourse. It is typically harmless, but is worth noting to your healthcare provider.

Cervical ectropion is a normal physical anomaly that people are born with. Hormonal changes can also cause it, making it more likely during adolescence, pregnancy, or when taking an estrogen-containing contraceptive like the pill.

Early Miscarriage

Early miscarriage, which can occur five weeks after ovulation, can cause vaginal bleeding.

Unlike implantation bleeding, early miscarriage bleeding typically causes:

• Heavier, darker blood flow
• Significant cramping
• Visible tissue and blood clots passing through the vagina

The pain and bleeding of miscarriage can be greater than during your period. Know, however, that bleeding doesn't occur with all miscarriages. Pain or cramping during pregnancy is enough to warrant medical attention.

Ectopic Pregnancy

Bleeding can also occur during an ectopic pregnancy—when a fertilized ovum implants outside of the uterus.

Symptoms of an ectopic pregnancy include:

• Irregular bleeding
• Cramps on one side of your body, ranging from mild to severe

These tend to occur six to eight weeks after your last period.

A Word From Verywell

Bleeding when suspecting a pregnancy can be alarming, but implantation bleeding is a common and normal early pregnancy sign.

It may be difficult to tell if the bleeding you experience is from implantation, your period, or another cause. Be sure to tell your healthcare provider about any bleeding you experience if you believe you may be pregnant, and seek emergency care if you experience abnormal heavy bleeding or pain if you know you are pregnant.