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What are Trimesters?

A normal, full-term pregnancy is 40 weeks, and can range from 37-42 weeks. It’s divided into three trimesters. Each trimester lasts between 12 and 14 weeks, or about three months.

Each trimester comes with its own specific hormonal and physiological changes. Being aware of the ways that your growing baby is affecting your body will help you to better prepare yourself for these changes as they happen. It’s also helpful to be aware of the specific risk factors (and associated medical tests) for each of the three trimesters.

First Trimester: Week 1 - Week 13

Second Trimester: Week 13 - Week 27

Third Trimester: Week 27 - Delivery

First trimester

Pregnancy date counting starts with the first day of your last normal menstruation cycle, and conception takes place in week 2. The first trimester lasts from the first through the 13th week of pregnancy. Although you may not look pregnant during the first trimester, your body is going through enormous changes as it accommodates a growing baby


In the first few weeks following conception, your hormone levels change significantly. Your uterus begins to support the growth of the placenta and the fetus, your body adds to its blood supply to carry oxygen and nutrients to the developing baby, and your heart rate increases. These changes accompany many of the pregnancy symptoms, such as fatigue, sickness, headaches, and constipation.

The first trimester is vital for the development of your baby. It’s best to take folic acid supplements the year before the pregnancy begins. The baby will develop all of its organs by the end of the third month, so this is a crucial time. It's important to maintain a healthy diet, including adding an adequate amount of folic acid in order to help prevent neural tube defects.

Avoid smoking and cut back on drinking alcohol. These habits, and any drug use (including some prescription drugs), have been linked to serious pregnancy complications and birth defects.

The first test that you will take during this trimester will most likely be an at-home pregnancy test that verifies that you are pregnant.

Your first doctor’s appointment should take place six to eight weeks after your last menstrual period. Your pregnancy will be confirmed by another urine test or a blood test. A Doppler machine will be used, or an ultrasound will be performed, to ensure that your baby has a heartbeat and to check on the baby’s health. Your doctor may also order a panel of blood work to check your immunity, nutritional levels, and indicators on the health of the baby.

During the first trimester, the risk of miscarriage is significant. If you’re taking prenatal vitamins and avoiding harmful substances, you’re already doing your baby a huge service and lowering your risk of miscarriage. Some doctors advocate cutting out caffeine. Deli meat and shellfish should be avoided in pregnancy, especially during the first trimester. These dietary changes are believed to help decrease the chances of miscarriage even further and help you stay healthy. The most important thing you can do for your baby is to be engaged in honest and direct communication with your healthcare provider about the choices you’re making, and follow their advice.

The first trimester is a good time to think about pregnancy, childbirth, breastfeeding, and parenting classes, and register for those in your community or online.

Second trimester

The second trimester (weeks 13-27) is the most comfortable period of time for the majority of pregnant women. Most of the early pregnancy symptoms will gradually disappear. You will likely feel a surge in energy levels during the daytime and be able to enjoy a more restful night's sleep.

Your abdomen will start to look pregnant, as the uterus will grow rapidly in size. It's a good time to invest in maternity wear, avoid restrictive clothing, and spread the good news of your pregnancy to your friends and family.

While the discomforts of early pregnancy should ease off, there are a few new symptoms to get used to. Common complaints include leg cramps and heartburn. You might find yourself growing more of an appetite, and your weight gain will accelerate. Work on gaining the amount of weight recommended and not putting on extra weight. Walk, choose healthy, nutrient-dense foods, and talk to your doctor about weight gain on each visit. Varicose veins, backaches, and nasal congestion may become apparent.

The second trimester is when most women can feel their baby move for the first time, usually by 20 weeks. The baby can even hear and recognize your voice during the second trimester.

Some screening tests may be performed in the second trimester. Be sure to talk to your doctor about your medical history, your family history, or genetic issues that could put you or your baby at risk


An anatomy ultrasound might be performed between weeks 18 and 22. At this scan, parts of the baby’s body will be measured and assessed to make sure that they are functioning. These body parts include the:

  • heart

  • lungs

  • kidney

  • brain

At the anatomy scan, you may be able to find out the sex of your baby. Let your doctor know if you would like to know, or if you would not like to know.

During the second trimester, doctors tend to test for gestational diabetes. Gestational diabetes can be detected between weeks 26 and 28 of pregnancy. During this test, you’ll be instructed to drink a high-glucose substance. After drinking it, you’ll be asked to wait an hour before having your blood drawn. This test will ensure that your body reacts properly to sugar during your pregnancy.

Third trimester

The third trimester lasts from the 28th week through to the birth of your baby. During the third trimester, you’ll start seeing your healthcare provider more frequently. Your doctor will regularly:

  • test your urine for protein

  • check your blood pressure

  • listen to the fetal heart rate

  • measure your fundal height (the approximate length of your uterus)

  • check your hands and legs for any swelling

Your doctor will also determine the baby's position and check your cervix in order to monitor how your body is preparing for childbirth.

Somewhere between weeks 35 and 37, you’ll be screened for bacteria called Group B streptococcus. A simple swab will be taken from your vaginal area before being sent away for lab evaluation. Group B strep, also called GBS, can pose a serious threat to newborns if it’s passed to them during your delivery. If you are GBS-positive, you’ll need to take antibiotics to prevent the baby from getting it.

Travel restrictions take effect during the third trimester. It’s advised that you stay in relatively close proximity to your doctor or midwife in case you go into labor early. Cruise ships typically will not allow women that are over 28 weeks pregnant to board. Airlines, though they do allow pregnant women to fly, advise that you do so only with permission from your healthcare provider.

The third trimester is a good time to educate yourself about labor and delivery. Take time out to enrol in a childbirth class. Childbirth classes are designed to prepare you and your partner for labor and delivery. It's a great way to learn about the different stages of labor and delivery options, and gives you the opportunity to ask any questions or voice any concerns to a trained childbirth instructor.

Due date

A normal, full-term pregnancy can last anywhere from 37 to 42 weeks. Your due date is really an estimated date of delivery (EDD). It’s dated from the first day of your last period, even though you actually conceive two weeks or so after this date. The dating system works well for women who have fairly regular menstrual cycles.

However, for women who have irregular periods, the dating system may not work. Other methods may be needed to determine the EDD. The most accurate method of determining the due date is an ultrasound in the first trimester, because early fetal development is fairly regular across pregnancies.


Pregnancy is a time unlike any other in your life. It’s important to see your healthcare provider regularly to ensure the best outcome. Babies born to mothers who receive regular prenatal care have much better outcomes than babies born to women who don’t. By taking your prenatal vitamins, attending every doctor’s appointment, and undergoing all recommended tests, you’re doing everything you can to give your baby a healthy start in life.

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