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First Trimester of Pregnancy
Updated on
March 6, 2024

First Trimester of Pregnancy

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First Trimester of Pregnancy

Welcome to the first trimester! Your pregnancy test is positive, the symptoms are kicking in, and you’re wondering what all is in store for you and baby over the next nine months. Most things are a long way off, but there’s still a lot that will happen this trimester.

You’ll probably be on a rollercoaster of emotions—all those pregnancy hormones hard at work making your baby can take you on a wild emotional ride. You also might be feeling some morning sickness (which really can last all day), not to mention really, really, really tired. That’s all part of the first trimester journey. Here’s what you can expect during those first 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, or until 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, so here’s more information on how to count your pregnancy weeks.

Baby’s development in the first trimester

During the first trimester, your baby grows from the size of a poppy seed (.05”) to as big as a roll of film (2.1”) by week 12.

While your body is busy making a home for baby (the amniotic sac, placenta and the umbilical cord), your baby is hard at work growing. At five weeks, their major organs start developing. “Three types of tissue layers are being separated and will determine specific types of organs and tissues usually based on location in the body and function,” says Dr. Kim Langdon, ob-gyn. “The early spinal cord, spinal bones and nerves start out as a neural tube that develops in the [outermost] layer, called the ectoderm. The middle layer (mesoderm) forms the heart, [which will be] divided into four chambers to pump blood. The endoderm [the innermost layer] forms the lungs, intestines, urinary, genitals, liver, thyroid, and pancreas.”

“Within the first eight weeks, the embryo develops rudimentary structures for all body systems—it’s at this point in gestation that one can observe a heartbeat,” says Dr. Kecia Gaither. If you see your healthcare provider around week six to week eight, you should be able to hear the heartbeat via ultrasound.

At 10 weeks, your baby is now a fetus and no longer an embryo. “The first trimester is when all the organs are formed. Then once everything is formed, the rest of the pregnancy is how the fetus fully develops,” says Dr. Sarah Yamaguchi. This means all the building blocks are in place, and your baby will keep getting bigger and bigger. By now, their eyes are fully formed, though they won’t open them for a while. And their teeth are in place, but you won’t see these little chompers until they’re about six months old.

By the end of your first trimester of pregnancy, baby might start sucking their thumb as they develop reflexes, their intestines are fully developed and their face is more babylike.

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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 pregnant people notice first.

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 serious 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 still feel ill well into their third trimester.

In addition to feeling queasy, many pregnant people 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, which can cause dehydration and weight loss and often requires treatment by a doctor. Be sure to talk with your healthcare provider if you’re throwing up multiple times a day for several days in a row.


During the first trimester, you’re pumping out more progesterone to build up the uterine lining and prevent miscarriage, and these higher levels can also can lower your blood pressure and blood sugar, all which you cause to experience 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
  • Make your bedtime an early one
  • 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 pregnant people get a burst of energy during the second trimester (though fatigue does 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.

Now’s a good time to invest in a comfortable, supportive bra (or three). You may find non-underwire bras feel better on sore breasts. Nursing bras or nursing camis can be good bets—buy ones that are 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 all over the place during your first trimester. Once you find out you’re pregnant, you may experience 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 mood swings.

You may feel overwhelmed at times, and that is 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.


If you’re feeling blocked up, know you’re not alone. 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.

Weight gain in the first trimester

You’ll probably gain about three to four pounds during the first trimester. Your baby is still very small, and, if you’ve been experiencing morning sickness and have trouble keeping food down, you may not gain that much. You may even lose a few pounds. As long as you’re seeing your healthcare provider, it shouldn’t be cause for worry. You’ll make it up in the second trimester.

First Trimester Pregnancy 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.
  • Buy prenatal vitamins. While these don’t replace a balanced diet, prenatal vitamins are packed with the extra 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.
  • Start creating your budget and savings plan for pregnancy 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 them 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.)


Babylist Staff

Babylist editors and writers are parents themselves and have years of experience writing and researching, coming from media outlets like Motherly, the SF Chronicle, the New York Times and the Daily Beast, and the fields of early childhood education and publishing. We research and test hundreds of products, survey real Babylist parents and consult reviews in order to recommend the best products and gear for your growing family.

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