pregnant woman looking at ultrasound trimester

What do I need to know about each trimester of pregnancy?

When you become pregnant, you hear talk about trimesters early and often. So, what are trimesters and what is the difference between the first trimester, the second trimester and the third trimester? Here is a trimester-by-trimester breakdown so you know what to expect as pregnancy progresses.

Pregnancy lasts 40 weeks, and it is broken up into trimesters of about 13 weeks each.

“Each trimester, we have specific goals that need to be accomplished,” said Vanessa Foster, MD, an OB/GYN for OSF HealthCare. “And each trimester is associated with its own risk.”

Every woman is different, and every pregnancy is different. That’s why it’s practically impossible to describe pregnancy in a way that completely matches everyone’s experience. But, there are symptoms of pregnancy and risks that are common as your baby develops.

First trimester

The first trimester starts the moment your baby is conceived and lasts until you are about 3 months pregnant.

First trimester symptoms

Common first trimester symptoms you can expect are breast tenderness, nausea and vomiting – also known as morning sickness.

How much weight should you gain in the first trimester? You will likely gain only one to four pounds.

First trimester care

As for your care, you’ll see your OB/GYN about every four weeks. You can expect to have plenty of blood drawn for prenatal testing as part of your first trimester screening. Genetic testing is done and so are vaginal cultures to make sure you don’t have any infections. You’ll also get a Pap smear if you haven’t had one in the past year.

A first trimester ultrasound is done to confirm your pregnancy dates. This may be when you can hear a little baby heartbeat. Around four to seven weeks of pregnancy, a heartbeat becomes detectable.

Being pregnant impacts your diet. Avoid heavily processed lunch meats because they can carry bacteria that cause food poisoning. The same goes for soft cheeses, like Brie and feta. There are also several fish you should avoid because they carry high levels of mercury. Mercury can harm your baby’s development.

Taking ibuprofen is also off the table for the entire pregnancy.

You should, however, begin taking a prenatal vitamin every day.

By the end of the first trimester, your baby will be about the size of a thumb. It will have started to form the neural tube, which develops into the brain and spinal cord. Your baby will also start to develop ears and eyes at the end of the first trimester.

Typically, you will not feel your baby moving during the first trimester, though you shouldn’t have to wait much longer.

Don’t hold off on seeing a physician once you believe you’re pregnant. Those first weeks and months of development are crucial.

“The first trimester is the most critical trimester,” Dr. Foster said. “That’s when most miscarriages occur because that’s when the critical early development happens. And if the development doesn’t occur correctly, it leads to miscarriage.”

Call your health care provider immediately if you’re experiencing any vaginal bleeding at all.

Also, call your provider if you’re having so much nausea and vomiting you can’t keep anything down, and you’re starting to feel weak. These symptoms could be signs that something is not right.

Second trimester

The second trimester starts at about week 13 and runs until week 26 or 27. This stage of pregnancy is often the easiest in terms of dealing with the physical impact of pregnancy.

Second trimester symptoms

“During the second trimester, a lot of women feel great because they’re not having nausea. And the vomiting resolves,” Dr. Foster said. “And they’re not so heavy that they’re uncomfortable.”

Early in the second trimester is usually when you feel your baby move. It usually happens at some point between week 14 and week 18. Between week 16 and week 24 is when you can commonly feel the baby kick from outside.

“It might be difficult to tell if that’s the baby moving, or if it’s your stomach growling. It’s a little hard to tell at first,” Dr. Foster said. “But as the pregnancy progresses, you’ll definitely be able to tell a frank kick from just your stomach rumbling.”

How much weight should you gain in the second trimester? You should expect to gain between a half pound and a pound per week.

Second trimester care

The second trimester is when you get a pregnancy ultrasound that includes an anatomy scan of the baby. You will likely be able to learn the sex of the baby during this ultrasound, if you want to know.

You’ll have a prenatal visit with your OB/GYN every two or three weeks. You’ll continue receiving ultrasounds and testing, but the testing spreads out in the second and third trimesters.

Your glucose test during pregnancy occurs during this trimester. This test checks to see if you’re at risk for gestational diabetes.

Once your baby starts moving, the little one shouldn’t ever really stop for too long. If they do, call your provider.

“Your baby is a person, and that person should move in there every now and then,” Dr. Foster said. “If your husband didn’t move for 12 hours, you wouldn’t wait 12 hours to tell somebody. So just keep in mind that your baby’s a person, and they do people things. They sleep, they get the hiccups, they move around.”

At the end of the second trimester, everything is formed pretty well in your baby’s anatomy, except the lungs. You may even be able to see your baby suck their thumb on an ultrasound.

Third trimester

The third trimester starts at 27 or 28 weeks pregnant and lasts until you deliver.

Third trimester symptoms

You can expect to become more uncomfortable because of the weight of your baby bump. Human beings are meant to stand up straight, but your center of gravity is off when you’re pregnant. The weight of the baby pulls you forward, so your back muscles tend to tighten up to hold you upright.

In addition to back pain, you may start to experience cramping in the third trimester.

How much weight should you gain during the third trimester? You should continue to gain between a half pound and a pound per week until you deliver your baby.

Third trimester care

The frequency of your OB/GYN appointments increases in the third trimester to once a week. These visits include a blood pressure check and a urinalysis.

During the third trimester, avoid traveling too far from where you’re going to deliver. Dr. Foster said she doesn’t like her patients to travel outside of the state after about 28 to 32 weeks.

You should feel your baby moving every hour. Call your provider if you perceive a decrease in your baby’s movement. Having watery discharge or vaginal bleeding are also reasons to call your provider.

By 37 weeks, your baby is considered fully formed and considered to have reached full term. If you notice any signs of labor before 37 weeks, call your provider. You may be going into birth prematurely.

Last Updated: June 14, 2023

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About Author: Ken Harris

Ken Harris is the proudest father and was a writing coordinator for the Marketing & Communications division of OSF HealthCare.

He has a bachelor's in journalism from the University of Wisconsin-Madison and worked as a daily newspaper reporter for four years before leaving the field and eventually finding his way to OSF HealthCare.

In his free time, Ken likes reading, fly fishing, hanging out with his dog and generally pestering his lovely, patient wife.

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Categories: Birth & Maternity