In the first trimester — the first three months of pregnancy — your body undergoes some pretty dramatic changes.

While you may still be able to fit into your regular pants, there’s a lot going on inside your body. This includes surging hormone levels and building a new blood flow system. With so much happening, first trimester bleeding is common.

According to one large 2009 study, 30 percent women have spotting or light bleeding in the first trimester. This can be a very normal part of early pregnancy. Many women experience some bleeding and go on to have healthy pregnancies.

There are several reasons why you might find vaginal spotting or bleeding in the first trimester. Let’s take a look at some of the common causes.

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Spotting or light bleeding is usually not anything to worry about, especially if it lasts for a day or two. One dated research study showed that women who have spotting and light bleeding in the first trimester have similar pregnancies to women who don’t bleed.

On the other hand, heavy bleeding and other symptoms may be indicators of more serious conditions.

Implantation bleeding

Implanting means the fertilized egg is busy making use of the space and burrowing into the side of your womb (uterus). This happens about 6 to 12 days after you’ve conceived. The fertilized egg floats into its new home and must attach itself to the uterine lining to get oxygen and nutrition.

This settling in can cause light spotting or bleeding. Implantation bleeding usually happens just before you expect your period to begin. In fact, this kind of bleeding is often mistaken for a light period.

Distinguishing between implantation bleeding and your period can be challenging. It doesn’t help that other symptoms are similar to PMS:

  • mild cramping
  • lower backache
  • headaches
  • nausea
  • tender breasts

But there are some clues that what you’re seeing isn’t a typical period. Implantation bleeding is usually lighter in color than a period — a light pink to a dull brown. It typically lasts from a few hours to a couple days and doesn’t involve heavy bleeding.

Cervical polyp

About 2 to 5 percent of women have polyps — small, finger-like growths — on the cervix, the gateway from the vagina to the uterus.

Cervical polyps are usually benign — they don’t cause cancer. However, they can get inflamed or irritated and lead to bright red bleeding. Or you may not have any other symptoms at all, but they’re easy to diagnose during a routine pelvic exam.

Intercourse or a physical exam

Speaking of pelvic exams, keep in mind that anything that might poke at or near the cervix can also irritate it and cause bleeding. Yes, this includes sex! This happens because pregnancy hormones may make your cervix — along with many other things — more sensitive than normal.

You might see bright red blood on your underwear shortly after sex or a physical checkup. Don’t fret! The bleeding usually happens once and then goes away on its own.

Miscarriage

Sometimes what begins as spotting or lighter bleeding becomes heavy bleeding. It’s true that any heavy bleeding in the first trimester, especially if you also have pain, might be linked to a miscarriage. Most miscarriages happen in the first trimester of pregnancy.

Up to 20 percent of all pregnancies are miscarried. You can’t prevent most miscarriages, and they’re definitely not your fault or a sign that something’s wrong with you. Most women can and do go on to have a healthy pregnancy and baby.

If you’re going through a miscarriage, you might have symptoms like:

  • heavy vaginal bleeding
  • bleeding that is bright red to brown in color
  • pain in the lower stomach
  • dull or sharp pain in the lower back
  • severe cramping
  • passing clots of blood or tissue

If you have any of these symptoms, call your doctor. You can have bleeding and other symptoms of a miscarriage without having miscarried. This is called a threatened abortion (abortion is a medical term here).

Causes of threatened abortion include:

  • a fall or trauma to the stomach area
  • an infection
  • exposure to certain medications

Carrying multiple babies

If you’re pregnant with twins (or another multiple of babies), you might have a greater chance of first trimester bleeding due to causes like implantation bleeding.

Miscarriages in the first trimester are also more common when you’re pregnant with more than one baby.

On the other hand, a 2016 study that followed more than 300 women who were pregnant with twins from in-vitro fertilization (IVF) found that they had a high chance of healthy pregnancies. Bleeding in the first trimester didn’t affect this.

Ectopic pregnancy

An ectopic pregnancy happens when the fertilized egg mistakenly attaches somewhere outside the womb. Most ectopic pregnancies are in the fallopian tubes — the connections between the ovaries and the womb.

An ectopic pregnancy is less common than a miscarriage. It happens in up to 2.5 percent of all pregnancies. A baby can only grow and develop in the womb, so ectopic pregnancies have to be medically treated.

Symptoms include:

  • heavy or light bleeding
  • sharp waves of pain
  • severe cramping
  • rectal pressure

If you have an ectopic pregnancy, know that there’s nothing you did to cause it.

Molar pregnancy

Another cause of bleeding in your first trimester is a molar pregnancy. This rare but serious complication happens in almost 1 in every 1,000 pregnancies.

A molar pregnancy or “mole” happens when the placental tissue grows abnormally due to a genetic error during fertilization. The fetus may not grow at all. A molar pregnancy can cause a miscarriage in the first trimester.

You might have:

  • bright red to dark brown bleeding
  • lower stomach pain or pressure
  • nausea
  • vomiting

Subchorionic hemorrhage

Subchorionic hemorrhage, or hematoma, is bleeding that happens when the placenta slightly detaches from the wall of the womb. A sac forms in the gap between the two.

Subchorionic hemorrhages vary in size. Smaller ones are the most common. Larger ones cause heavier bleeding. Many, many women have hematomas and go on to have healthy pregnancies. But a large subchorionic hemorrhage may also increase the risk of a miscarriage in the first 20 weeks of pregnancy.

Symptoms include:

  • light to heavy bleeding
  • bleeding may be pink to red or brown
  • lower stomach pain
  • cramping

Infection

Bleeding in the first trimester might have nothing to do with your pregnancy at all. An infection in your pelvic area or in the bladder or urinary tract can also cause spotting or bleeding. They may be caused by bacteria, viruses, or fungi.

A serious yeast infection or inflammation (vaginitis) can also cause bleeding. Infections typically cause spotting or light bleeding that is pink to red in color. You may have other symptoms like:

  • itching
  • lower stomach pain
  • burning when urinating
  • white discharge
  • bumps or sores on the outer part of the vagina

Bleeding in the second or third trimester of your pregnancy is normally more serious than first trimester light bleeding.

Causes include:

  • Cervix problems. Inflammation or growths on the cervix can cause light bleeding. This is usually not serious.
  • Placental abruption.The placenta detaches from the womb wall before or during labor. This happens in just 1 percent of pregnant women.
  • Placenta previa. The placenta is too low in the uterus and partly covers the cervix. Bleeding happens without pain.
  • Vasa previa. Some of the placenta’s blood vessels go across the cervix.
  • Premature labor: Bleeding may mean that you’re in labor too early.
  • Missed abortion. A miscarriage may have happened earlier without any signs.

Let your doctor know if you experience any kind of bleeding during pregnancy. Get immediate medical care if you have any of these symptoms:

  • heavy bleeding
  • discharge with clots or tissue
  • severe pain
  • intense cramping
  • severe nausea
  • dizziness or fainting
  • chills
  • fever of 100.4°F (38°C) or higher

What your doctor will look for

A quick examination can usually tell your doctor what is causing your bleeding. You may need:

Your doctor will likely also look at pregnancy markers. A blood test looks at your hormone levels. The main hormone in pregnancy — made by the placenta — is human chorionic gonadotropin (hCG).

Too much hCG can mean:

  • twin or multiple pregnancy
  • molar pregnancy

Low levels of hCG may mean:

  • ectopic pregnancy
  • possible miscarriage
  • abnormal growth

Scans can show where the developing baby is and how it’s growing. The baby’s size can be measured on an ultrasound. The heartbeat can be checked with the ultrasound or Doppler scan as early as five and a half weeks of pregnancy. All these checks can reassure you and your doctor that everything is just fine.

Some issues that cause first trimester bleeding, like a cervical polyp, may be treated right in your doctor’s office. Other issues may need more treatment, medication, or surgery.

If the bleeding is a sign that your pregnancy can’t safely continue, your doctor may prescribe medications such as:

  • Methotrexate is a drug that helps your body absorb harmful tissue like in an ectopic pregnancy.
  • Misoprostol is used to end a dangerous pregnancy in the first 7 weeks.

You’ll need follow-up appointments to check on your health. Your doctor will make sure there is no leftover tissue or scarring in your womb. Your doctor can advise when it’s safe to try to conceive again if that’s what you want.

A miscarriage at any point in your pregnancy is a loss. Talking to a therapist or counselor can help you and your partner grieve in a healthy way.

Bleeding in your first trimester can be alarming. But in most cases, spotting and light bleeding are just a normal part of early pregnancy.

Heavy bleeding may be a sign of something more serious. You should always see your doctor if you have any questions or concerns regarding bleeding.

Causes of first trimester light bleeding and spotting that are usually not harmful to you and your baby include:

  • implantation
  • cervical polyps
  • uterine infections
  • yeast infection
  • carrying multiple babies

More serious causes of bleeding in the first trimester are:

  • miscarriage
  • threatened abortion
  • molar pregnancy
  • ectopic pregnancy
  • subchorionic hemorrhage (in many cases, women go on to have healthy pregnancies)

Pregnancy can be a roller coaster of emotions and symptoms. Above all, keep people you love and trust in the loop. Having someone to talk to about what you’re going through — even if your symptoms are completely normal — can make the experience much easier.