Fatigue, Nausea, and Swollen Breasts: First-Trimester Pregnancy Symptoms Explained

Since most pregnancy symptoms don't start to happen until around the fourth or fifth week of pregnancy, it can be hard to tell the difference between early pregnancy symptoms and signs that your period is on the way. As the weeks go on, however, the symptoms become clearer. Here's a breakdown, according to our experts.

Week 1: At this stage, your period isn't late yet; in fact, your cycle would have just ended, and you're now waiting for ovulation.

Week 2: Ovulation occurs, and during this time, you might feel slight twinges on the left or right side of your pelvis. According to Cleveland Clinic, only around 40 percent of people who ovulate are able to feel those twinges.

Week 3: Your fertile window has passed, and if sperm has found an egg, the ovum will be traveling to the uterus. Some people can feel very early symptoms at this stage, including implantation bleeding, which happens when the fertilized egg attaches to the lining of the uterus, Mayo Clinic explains. You may also experience a heightened sense of smell, nausea or mild queasiness, and breast tenderness, according to NY Presbyterian.

Week 4: Your period is likely late, and you should be able to take a home pregnancy test. You might be feeling extra tired or have waves of nausea. Tender breasts and abdominal pressure are also common.

Weeks 5-6: Nausea or morning sickness might be more consistent, and feeling tired will likely continue.

Weeks 7-9: You might experience ongoing nausea and fatigue, plus bloating, constipation, sore breasts, and heartburn.

Weeks 10-12: Many of the symptoms from the weeks before will likely continue, though nausea and fatigue may start to ease up.

Understanding Bloating in the First Trimester

For many people, one of the first and most obvious symptoms of pregnancy in the first trimester is bloating. It's also another symptom that can show up even before missing a period.

"Bloating is a very common symptom of pregnancy," Dr. Wetter shares. "This is due to the hormone progesterone, which increases in early pregnancy in order to support the fetus as it grows. A side effect of the increased progesterone is it relaxes muscles, including the muscles in your GI tract. This can slow digestion, which can cause gas to be trapped and constipation."

Weird Pregnancy Symptoms You Might Not Expect

So many fast changes happen in the first trimester, and hormones largely drive those early changes, according to Johns Hopkins Medicine. These hormones, including human chorionic gonadotropin (hCG) and progesterone, spike during the first trimester and drive many necessary changes for the pregnancy to develop.

"HCG hormone levels found in the mother's blood and urine rise a lot during the first trimester," Johns Hopkins Medicine notes. "They may play a part in the nausea and vomiting often linked to pregnancy."

And the other main player, progesterone, rises very early in pregnancy, according to Johns Hopkins Medicine. "This hormone is made by the ovaries and by the placenta during pregnancy," the site explains. "It stimulates the thickening of the uterine lining for implantation of a fertilized egg."

Another early pregnancy hormone called human placental lactogen (HPL) "is a peptide hormone produced by the placenta. It is detectable as early as 6 weeks' gestation and increases across gestation, peaking at around 30 weeks," according to a 2022 study published in BMJ Open.

With all those hormones surging, experts believe they play a big part in the development of pregnancy symptoms, including some symptoms beyond nausea and fatigue. According to NHS, symptoms during the first trimester that you might not expect may include feeling dizzy, getting nosebleeds, experiencing blurry vision, having bleeding gums, and tasting metal.

Pregnancy can be confusing, especially during those early few weeks. It's always good practice to reach out to your care providers if you're concerned or worried about any of your pregnancy symptoms.