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If you are a pregnant mother or involved father and sign up for classes at the PHI Center in anticipation of your baby’s birth, we’ll put you a class based on your trimester. We offer classes for first, second or third trimester parents so you’ll learn specifically about what’s happening in your pregnancy. If you’ve been seeing a doctor you’ve probably heard the word “trimester” spoken dozens of times, but do you know what it means? Do you know how we got that term and why we use it? Let’s learn together.

The American school system made popular the word semester. Most schools divide their academic year into two “semesters,” but how does that help us understand a trimester? The answer’s found in the etymology of the word.

Technically, semester is a German word derived from its older Latin cousin “semestris.” That word comes from two smaller Latin words, “sex” which means six, and “mensis” which means month. In the German school system Semester is used literally; they have two six-month semesters. In America the semester doesn’t represent six months, instead it represents dividing one period of time into two equal parts (The academic semester is closer to four months). For a pregnancy it’s a little simpler, trimester comes from two words as well: “tri” meaning three, and “mensis” meaning month. Trimester literally means three months. As an aside, the term menstrual also comes from this Latin word “mensis,” and simply refers to a monthly cycle. Your pregnancy is divided into three, roughly three-month periods called trimesters.

The key to understanding trimesters and your due date is accepting it’s not an exact science. The 280 day (forty week) gestation period is derived from an average. The due date of a baby has a give or take of up to two weeks based on when conception happened. And we can’t forget some months have five weeks while others have four. At the Phi Center, we’ll utilize your last menstrual period (LMP) to estimate your due date and calculate which trimester you’re in.

We offer first trimester classes up to the twelfth or thirteenth week of your pregnancy, second trimester classes until your about twenty-six weeks, and third trimester classes up till your baby is born. If the first day of your LMP was January 1st you would enter your second trimester around March 25th, enter your third trimester about the first of July, and possibly deliver on October seventh. But remember the key, these dates are estimates not exacts. You might deliver at the end of September or the middle of October, but that’s okay. And you’ll be okay, just listen to your doctor and enjoy the process.

So a trimester is roughly one third of your pregnancy. The exact day you grow from one trimester to another varies depending on when the little life you’re expecting began.

A normal, full-term pregnancy can range from 37-42 weeks and is divided into three trimesters. Each trimester lasts between 12 and 14 weeks.


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 fetus.

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, morning sickness, headaches, and constipation.

The first trimester is vital for the development of your baby. The fetus 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. Cut out any bad habits, such as smoking and alcohol. Both have been related to serious complications in pregnancy and birth defects.


The second trimester (weeks 13-27) is often the most comfortable period of time for the majority of pregnant women. Most of the early pregnancy symptoms will gradually disappear, and you should enjoy a more restful night’s sleep and a surge in energy levels during the daytime.

Your abdomen will start to look pregnant, as the uterus will grow rapidly in size. At the end of the second trimester, your baby will be almost four times as big as it was at the end of the first trimester. It’s a good time to invest in maternity wear, and spread the good news of your pregnancy with 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.

Screening tests are also performed in the second trimester, and this is when a diagnostic test would be performed. Be sure to talk to your doctor about your medical history and any issues that could put you or your baby at risk.


The third trimester lasts from the 28th week through the birth of your baby. During the third trimester you will start seeing your health care 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.

The third trimester is a good time to educate yourself about labor and delivery. Take time out to enroll 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.


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 and 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 all pregnancies.