The earliest reason you may bleed during a pregnancy is implantation bleeding, which occurs when a fertilized egg attaches to the uterine lining, per What to Expect. It happens six to 12 days after fertilization, and the bleeding is typically light. In fact, people often mistake implantation bleeding as a normal period, according to Mayo Clinic, which might lead to an incorrect prediction of your due date.

Implantation bleeding is an example of what Dr. Karkowsky calls 'benign' bleeding: "It usually doesn't mean anything terrible is going on!" But she adds that "any time you have new vaginal bleeding in pregnancy, it's always worth getting evaluated by a provider who is comfortable with women's health and early pregnancy care." So make an appointment with your doctor just to be sure.

Another common reason for bleeding during the first trimester is a pap smear. Even non-pregnant women may experience bleeding after a pap smear, but the possibility increases when you're pregnant because "your cervix develops additional blood vessels during pregnancy," according to Healthline. It's nothing to worry about, and it will likely stop on its own after a few hours. "Most of the time, this bleeding is light and not worrisome; but any new vaginal bleeding should be evaluated," says Karkowsky. And since you'll be at the doctor anyway if you're getting a pelvic exam, they can make sure you're all good.

One of the more dangerous causes of bleeding in the first trimester is an ectopic pregnancy. In a normal pregnancy, a fertilized egg travels from the fallopian tubes to the uterus, where the zygote will grow into a baby. But in an ectopic pregnancy, the zygote starts to develop outside of the uterus, per the American College of Obstetricians and Gynecologists. "These pregnancies are usually in the woman's fallopian tube, though they can also be on the ovary or in the cervix, or even more rarely elsewhere," Karkowsky tells Romper. As the pregnancy develops, it will cause pressure in the fallopian tube, ultimately leading it to burst, which will cause major bleeding. They occur in about 1-2% of pregnancies, according to Karkowsky.

A burst fallopian tube can cause life-threatening bleeding and must be treated with immediate surgery, so you should see a doctor as soon as possible if you experience heavy, unexpected bleeding when you're pregnant, especially if it's accompanied by cramping, feeling faint, or shoulder pain, per The American College of Obstetricians and Gynecologists. The College also warns that, unfortunately, there is no way an ectopic pregnancy can be carried to term, but a doctor can catch it in the early stages and prevent the need for surgery with medicine that will end the pregnancy instead, which is one of the reasons you should see a doctor as soon as you can if you think you are pregnant.

Another more serious reason for first trimester bleeding is a molar pregnancy. In a molar pregnancy, a fertilized egg grows into an abnormal mass or a tumor as a result of chromosomal issues, as WebMD explains. The fertilized egg cannot survive in a molar pregnancy. A molar pregnancy will often shed itself, with bleeding being the first symptom. That bleeding would occur within the first three months of your pregnancy. If a doctor detects a molar pregnancy before you bleed or display any other symptoms, they may remove it.

Only a healthcare professional can detect a molar pregnancy, which is again why it's so important to make an appointment with an OB-GYN as soon as you suspect you are pregnant. Plus, Karkowsky adds that "Rarely, a molar pregnancy can later lead to certain types of cancers called gestational trophoblastic neoplasias; so any woman who has had a molar pregnancy needs to be followed closely to make sure she's OK, especially before she gets pregnant again."

As Medical News Today explained, subchorionic hemorrhages happen when blood gathers between the uterus and the gestational membrane. While it can happen during the first trimester, Deena Blumenfeld — a registered prenatal yoga instructor, certified Lamaze instructor and childbirth educator — says that it "is more common in the second trimester and affects 1.7% of women."

The bleeding will typically be light, but because there are so many reasons you can bleed during pregnancy, it's still best to check with your doctor to make sure you and the baby are OK. Plus, a study published in Proceedings in Obstetrics and Gynecology found links between subchorionic bleeding and early delivery and miscarriages, so you should see your provider if you have a lot of spotting.

Bleeding after sex is very common during pregnancy, especially if the bleeding is light. It can be accompanied by some light cramping too, and both symptoms should dissipate after a few hours. It's similar to the spotting you could experience during a pap smear. As Karkowsky explains, "The whole pelvic area has more blood flow, and changes to the cervix can make some of the small blood vessels more exposed." So any spotting during sex is likely because you're irritating those blood vessels. But she adds that "any new bleeding during or after sex (whether in pregnancy or outside of it!) should be checked out fully, so that we can make sure it's not anything else concerning."

Lastly, bleeding during the first trimester could be a sign of a miscarriage. Often, miscarriages happen so early in a pregnancy that you may not even realize you were pregnant, though between 15 to 20 percent of recognized pregnancies will result in a miscarriage per WebMD. Heavy bleeding is the most common sign, and it is often accompanied by cramping. You should see a doctor as soon as possible if you believe you're miscarrying, or if you're unsure of why you're bleeding.

The reasons for bleeding during the first trimester vary, but the bottom line is that only a doctor can truly diagnose the cause, so it's best to check with a professional even if you're fairly certain the reason for your bleeding is benign.

If you do experience bleeding at any point in your pregnancy, remember it's not your fault. As Blumenfeld says, "There is a pervasive myth that women can do or not do things to cause harm to her pregnancy. Which leads to the worse incorrect myth that a woman can cause a miscarriage... Nothing a woman does or doesn't do will cause bleeding, nor is there anything she can do to prevent it."

Editor's Note: This post has been updated from its original version.