First Trimester of Pregnancy

Your baby’s earliest growth and your first symptoms: what to expect in weeks 1–12 of pregnancy.

By Amylia Ryan

Updated On Oct 9, 2025

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In This Article

How Long Is the First Trimester?

Your Baby In the First Trimester

Your Body In the First Trimester

First Trimester Pregnancy Symptoms

Increased vaginal discharge

Headaches

Top Tip for the First Trimester

✅ First Trimester Checklist

Frequently Asked Questions About the First Trimester of Pregnancy

Welcome to the first trimester, the very beginning of your pregnancy! You got a positive pregnancy test result, the early symptoms are setting in and you may already be wondering what’s ahead for you and your baby over the next nine-ish months. While the due date feels a long way off right now, there’s already a lot happening.

Pregnancy hormones are working overtime to kick off your baby’s growth, and they can take you on a wild ride, both physically and emotionally. You may feel waves of nausea throughout the day (morning sickness can actually hit anytime, day or night) and notice odd aches and pains, not to mention you may be really, really, really tired. That’s all part of the first trimester journey. We talked with ob-gyns Dr. Tina Hendrick, Dr. Sarah Yamaguchi and Dr. Suzy Lipinski about what you can expect during these first 12 weeks.

How Long Is the First Trimester?

The first trimester of pregnancy lasts from week 0 (the start of your last menstrual period) through the end of week 12 (when you’re three months pregnant). That’s right, you’ll count your pregnancy weeks starting from before your baby was even conceived. It’s a little bit complicated—here’s more information on how to count your pregnancy weeks.

This early stage includes some big milestones: getting a positive pregnancy test, establishing prenatal care with your chosen provider and seeing your baby for the first time at your first ultrasound appointment.

Your Baby In the First Trimester

Even though your baby is still tiny, so much is happening during these first 12 weeks. Your baby grows from the size of a poppy seed (.05”) to as big as a Tamagotchi (2.1”) by week 12. These are the biggest developmental milestones happening along the way (if you'd like to have each week broken down, check out our pregnancy week by week pages):

Weeks 3–4: Implantation! The blastocyst (teeny tiny collection of cells that will eventually be a whole baby) secures itself to the wall of your uterus.
Week 5: Major organs start to form, including the brain, heart, stomach, liver and kidneys.
Weeks 6–7: The heart starts working, limb buds (the earliest signs of arms and legs) begin to sprout, and the placenta, amniotic sac and umbilical cord are forming.
Week 8: Facial features are developing, as well as tiny webbed fingers and toes (they’ll lose their webbing in a couple weeks). The embryo also starts moving right around now, though you can’t feel it yet.
Week 9: Bones and muscles begin forming, though bones are starting out as cartilage at first. The eyes are now fully formed but will stay closed for a little while longer.
Week 10: Baby is no longer considered an embryo and is now officially a fetus. All major organs are present, though many are still in the critical development stage.
Week 11: Genitals are now differentiated, but are still too small to see clearly on an ultrasound.
Week 12: Baby has doubled in size over the past three weeks! Intestines are fully formed and facial features have finished developing.

Your Body In the First Trimester

The average weight gain in the first trimester is about three to four pounds, but don’t worry if you’ve gained more than that or even lost weight—your body is going through a lot. “The most common reason for weight loss in early pregnancy is morning sickness,” Dr. Hendrick says. “A lot of times, [you] may not be nauseated or vomiting but just not have an appetite or aversions to certain foods.” As long as you’re regularly checking in with your provider at your prenatal appointments, they’ll make sure everything is on track as far as what’s healthy for your pregnancy.

By the time you reach the end of the first 12 weeks, you likely aren’t showing yet, but discomfort from symptoms (which we’ve outlined below) might be a good excuse to invest in loose, comfy maternity clothes a little early.

First Trimester Pregnancy Symptoms

“The most common early pregnancy symptoms are fatigue, breast tenderness, moodiness, constipation and nausea,” Dr. Yamaguchi says. Of course, pregnancy affects every body differently, and what you experience might be different from someone else you know. Symptoms are even different for the same person each pregnancy.

Morning Sickness

Morning sickness is one of the most common and most noticeable signs of early pregnancy. Aside from the first missed period, it’s often the symptom most people notice first when pregnant.

Thanks to rising levels of pregnancy hormones, including progesterone, human chorionic gonadotropin (hCG—the one that triggers the “plus” sign on your pregnancy test) and estrogen, you may find yourself dealing with nausea and vomiting morning, noon and night.

For most people, morning sickness begins around the fifth or sixth week of pregnancy and peaks around week nine. It’s typically gone by your second trimester—though some people may still feel ill well into their third trimester.

In addition to feeling queasy, many also find themselves with strong aversions to certain food or smells. “Eating small, frequent meals can help,” Dr. Yamaguchi says. For more tips on how to survive morning sickness, check out these hacks for morning sickness.

Most of the time, morning sickness is no cause for alarm. But if your symptoms are severe, it could be a condition called hyperemesis gravidarum. "HG is diagnosed when a patient becomes dehydrated and/or loses more than five percent of pre-pregnancy body weight," Dr. Lipinski says. "In severe cases, patients may need to be seen in the office or even admitted to the hospital for IV fluids and IV anti-nausea medications." Be sure to talk with your healthcare provider if you’re throwing up multiple times a day for several days in a row.

Fatigue

During the first trimester, you’re pumping out more progesterone to build up the uterine lining and prevent miscarriage, but these higher levels can also lower your blood pressure and blood sugar, which may cause first-trimester fatigue. There’s a good chance you’ve never been so tired! “There is not a lot you can do about fatigue, other than rest,” Dr. Yamaguchi says. “Even then, I find most patients are still tired.”

This fatigue can make it hard to function, so try to make sleep a priority. Here are a few tips:

Take naps whenever possible
Have an early bedtime
Keep your bedroom dark and cool
No screentime while in bed

The good news is that this exhaustion, like morning sickness, should pass after the first trimester. In fact, many people get a burst of energy during the second trimester, though fatigue does typically return during the third trimester as your baby gets bigger and your body gets ready for labor.

Tender breasts

Notice anything different about your breasts lately? They may feel bigger, sore and sensitive to the touch. That’s those pregnancy hormones at work again.

While the tenderness will subside after your first trimester, your breasts will continue to grow and change throughout your pregnancy in preparation for breastfeeding.

“For breast tenderness, I recommend making sure your bra is fitting you well and is supportive,” Dr. Yamaguchi says. So now’s a good time to invest in a comfortable bra (or three). You may find non-underwire bras feel better on sore breasts. Nursing bras or nursing camis can come in handy while you're still pregnant (we've linked to one below). Buy ones that are one size bigger or have extra hook-and-eye closures so you can continue wearing them throughout your pregnancy and postpartum.

Your emotions

Your emotions may feel a little all over the place during your first trimester. Once you find out you’re pregnant, you may feel a combination of excitement and joy, fear and nervousness. Add to the mix all those hormones, fatigue and morning sickness, and it’s understandable why you’re likely to have some mood swings.

You may feel overwhelmed at times, and that’s perfectly normal. Even if you’re really excited about being pregnant and welcoming a new baby, it’s natural to be worried about your baby’s development, how your life is going to change and all the other things that come along with having a baby.

Now’s the time to be good to yourself. Self care—naps, taking walks, reality TV marathons or whatever makes you happy—can help you feel like yourself, even if you’re bursting into tears at the slightest thing. Talk with other parents, your friends or your partner about how you’re feeling.

If you’re unable to feel any joy or are feeling hopeless, be sure to talk to your healthcare provider. They can help you find the support you need.

Bloating

Hormones slow down digestion in early pregnancy, which can leave you feeling bloated. That’s one main reason why you might want to start your maternity wardrobe now, even before your bump shows, since wearing loose clothing can help. Staying hydrated and lightly active can also keep the bloating at bay—just make sure that any exercise you’re doing is cleared by your provider.

Constipation

With that same digestive slow-down, constipation is very common during the first trimester. Up your fiber intake with foods like black beans, green veggies and whole wheat. And don’t forget to drink plenty of water.

The need to pee

You may find yourself running to the bathroom way more often than you used to. As your hormones fluctuate (notice a theme here?), you’ll have to go—a lot. Despite the frequent need to pee, it’s important to stay hydrated, though you might want to limit how much you drink before bed to avoid getting up all night long.

Increased vaginal discharge

A thin, milky discharge called leukorrhea is common during the first trimester. It’s your body’s way of preventing infection. It’s totally normal as long as it’s not discolored (pink, red, green or dark yellow) or foul-smelling. (Maybe just wear panty liners until it eases up.)

Headaches

Hormonal changes and increased blood volume can lead to mild headaches. Be sure to get plenty of rest and stay hydrated, and talk to your provider before taking any pain medication.

Top Tip for the First Trimester

Listen to your body and rest when you need to. Fatigue in the first trimester is real, and prioritizing sleep, naps and general downtime will help you cope with all the other symptoms going on.

✅ First Trimester Checklist

Download this checklist here!

First Trimester Checklist

Take a pregnancy test (and another one!) to make sure you’re really pregnant.
Check your health insurance to see what prenatal care and birth costs are covered.
Choose a healthcare provider.
Make your first prenatal appointment.
Start reading a pregnancy book or two. There’s a lot going on right now, and a little expert guidance can really help. A pregnancy journal can also help you keep track of all your milestones, symptoms and appointments.
Start taking prenatal vitamins (with your doctor’s guidance). While these don’t replace a balanced diet, prenatal vitamins are packed with the essential nutrients—like folic acid, iron and calcium—needed for pregnancy.
Research which, if any, prenatal tests you might opt for during your pregnancy.
Get rest. Take naps, go to bed early, get down time whenever you can. Do what you need to do to make it through this trimester.
Start creating your budget for baby gear and the first year of baby’s life.
Schedule your appointment for your 12-week ultrasound/nuchal screening test.
If you’re working, research your employer’s maternity leave policy and plan how to tell work you’re pregnant.
Your pants may be feeling tight toward the end of your first trimester. Start shopping for some maternity clothes. (Hint: leggings are a lifesaver.)

Frequently Asked Questions About the First Trimester of Pregnancy

When will my nausea and fatigue go away?

Many people notice both of these symptoms easing around weeks 12–14 as hormone levels level off.

When will I start to show?

It varies, but many people won’t show a baby bump until the second trimester, around between weeks 16–20. When and how much you show largely depends on your unique physical build as well as whether this is your first pregnancy (in which case you’ll likely show later).

When can I hear my baby’s heartbeat?

You may be able to hear a heartbeat on an ultrasound as early as week six, but more likely it’ll be closer to 10–12 weeks, at which point your providers may let you hear it with a Doppler device.

When should I tell people I’m pregnant?

It’s totally up to you. Some people wait until the end of the first trimester, when the risk of miscarriage decreases, while others share sooner.

First Trimester of Pregnancy

Your baby’s earliest growth and your first symptoms: what to expect in weeks 1–12 of pregnancy.

By Amylia Ryan

How Long Is the First Trimester?

Your Baby In the First Trimester

Your Body In the First Trimester