Your pregnancy: The first trimester

Congratulations! You’re at the very beginning of the roller-coaster ride of pregnancy. It's a time for all kinds of changes—big and small, thrilling and weird—and plenty of questions, too. Here’s what to keep in mind as you move through the first trimester. Learn about the stages of your pregnancy with our detailed week-by-week guide, detailing how your body is changing, plus the growth and development of your baby.

When does the first trimester start? When does it end?

Pregnancy math can be a little strange: The start of your pregnancy is calculated starting on the first date of your last period, not when conception or implantation happened. In other words, if you had your period four weeks ago, then ovulated and had sex two weeks ago, you’re four weeks pregnant. There are a number of different ways to figure out your due date, too.

The first trimester lasts until the final day of week 12.

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What are some of the pregnancy symptoms in the first trimester?

First-trimester symptoms are the subject of many jokes and plot twists in TV shows and movies, but every woman has her own experience in real life. You may have some, all or none of these symptoms, and that’s just fine.

Bleeding and cramps

Some women have light spotting (pinkish, reddish or brownish spots on toilet paper or underwear) or mild, menstrual-like cramps in the early days around week three (five to 12 days after conception). This is called implantation bleeding because the embryo that will become your baby is burrowing into the lining of your uterus. The spotting can last a day or several days.  Mild stomach cramps (often described as a slight tugging sensation that’s milder than period cramps) may kick in around week 10, when your uterus begins to stretch.

Dizziness

Human chorionic gonadotropin (hCG) is what triggers a positive pregnancy test. As this pregnancy hormone increases, it can lead to a whole range of symptoms that may start around week four but appear later on in the first trimester, too. Shifting hormones and a faster heartbeat that pumps more blood through the body mean that your blood pressure can dip in early pregnancy, leading to dizziness and light-headedness. Dizziness, or “feeling faint,” can also be triggered if you’re lacking food or water, feeling queasy, unable to eat or finding it hard to keep food down.

Morning sickness

About 80 percent of women experience morning sickness (which can happen at any time of day, actually) in their first trimester, often starting around week six. If you’re part of that particular club, you have surging hormone levels to thank. A stronger sense of smell can contribute to the queasiness, too, which may have developed to help early humans avoid spoiled food during pregnancy. Some women also find that they experience an odd metallic taste in their mouth, which may last for the first trimester or throughout pregnancy.

Morning sickness, also known as nausea and vomiting in pregnancy (NVP), isn’t any fun, but a bit of nausea is actually considered a good sign that your pregnancy is moving along normally. Talk to your healthcare provider if your symptoms are severe. They generally ease off around the end of your first trimester or at the beginning of your second trimester. Increasing levels of progesterone can also mean that you’re feeling gassy, bloated or constipated too (oh good) beginning around week 10.

Fatigue

Feeling incredibly drained and sleepy is often part of early pregnancy thanks to those hormones—your progesterone levels are high, which decreases your blood sugar and blood pressure levels and makes you feel tired. There’s also the fact that you are creating a tiny human from scratch. For a lot of women, the fatigue peaks around week nine and begins to dial back a bit.

Bathroom trips, sore breasts and acne

By week seven, your uterus has doubled in size and is pressing on your bladder, so you may have to pee all the time. Plus, those trusty hormones are stimulating your kidneys to produce more pee. Tender, slightly bigger boobs are another sign of early pregnancy. And how’s your skin? That hormone cocktail can cause acne to kick in. Your skin may settle down later on or you may have some pimples throughout your pregnancy.

Pregnancy emotions

Aside from all the physical changes, you should pay attention to your emotions, too. Yes, hormones can do a number on your mood. All. The. Feelings. It can be quite intense, but this life-changing thing is a big freaking deal. Excitement, joy, fear, guilt and anxiety can all be on board with your baby. If you’re not widely sharing your pregnancy news yet, it can feel a bit isolating, too. Make sure to reach out to your partner, family, friends or healthcare provider.

RELATED: 6 weird pregnancy symptoms that are actually totally normal

What kinds of medical tests and screenings take place in the first trimester?

At your first prenatal appointment, where you suspect (or know) that you’re pregnant, your healthcare provider may order a blood test that measures hCG levels to confirm your pregnancy and check that your hormone levels are on track.

Between eight and 13 weeks, you may be offered an appointment for a dating ultrasound to get measurements that can help determine a more accurate due date. You can probably hear your baby’s heartbeat (so exciting!) at 12 weeks, when your healthcare provider uses a handheld ultrasound called a Doppler.

Between weeks 11 and 14, you may opt for genetic screening (though many women don’t) through a blood test or a combination of a blood test and an ultrasound, also called a nuchal translucency (NT) scan. An NT scan measures the fluid and tissue at the back of your baby’s neck and checks for chromosomal abnormalities, such as Down syndrome. If your doctor or midwife recommends further screening, it will be done early in your second trimester.

Cannabis, alcohol and caffeine during pregnancy

If you’re considering using cannabis to deal with early-pregnancy symptoms (such as morning sickness) or as part of your regular routine, read up on some new studies—it’s generally not recommended due to links to low birthweight and other indicators of baby health. As you probably know, it’s time to pass on drinking alcohol, too, but you can still have coffee or tea (whew!) because drinking two small cups of brewed coffee or three cups of tea daily isn’t harmful.

Baby bump

Your bump probably hasn’t quite popped yet (or maybe it has!). By the end of your first trimester, you’ll probably notice that your lower abdomen feels firmer due to your growing uterus, so your pants may be tight. You’ll probably have to pick up a few stretchy yet supportive bras to accommodate those newly bigger boobs, too.

Pregnancy dreams

Having bizarro, vivid dreams? That’s because you’re waking up more (because you have to pee or aren’t comfortable), so you’re more likely to have rapid eye movement (REM) sleep, the resting phase where you’re more likely to remember your dreams.

RELATED: 7 early pregnancy signs: Heartburn, gas, tender breasts, and more

What’s happening with baby?

Your little one goes through an incredible transformation in the first trimester, growing from a poppyseed-size embryo at four weeks to a raspberry-size fetus (with a billion cells!) at eight weeks to a two-inch-long fetus at 12 weeks. Along the way, your babe builds all their muscles, limbs, organs, sex organs and bones. Their facial features are becoming more distinct, but their eyes will stay fused shut at this stage. Rubbery cartilage makes up the skeleton and is just beginning to harden into bone, and fingernails and hair are starting to grow. Toward the end of the first trimester, your baby can move their arms, legs and hands and their organs are starting to function. (Whew, no wonder you’re exhausted with all that going on!)

Read more: Your pregnancy by week How pregnancy hormones affect your body in each trimester