pregnant woman pouring orange juice

What is the first trimester of pregnancy and what happens to your body?

pregnancy lasts for about 40 weeks. The weeks are grouped into three trimesters. It starts on the first day of your last period – before you’re even actually pregnant – and lasts until the end of the 13th week. Today we will discuss what changes you can expect during the 1st trimester of your pregnancy.

A woman’s body goes through many changes during the first 12 weeks of a pregnancy. Women often start to have concerns over:

  • what to eat
  • which types of prenatal tests they should consider
  • how much weight they might gain
  • how they can make sure their baby stays healthy

Understanding what happens during each trimester of your pregnancy, can help you make informed decisions.

The first trimester is the earliest phase of pregnancy. It’s a time of rapid changes for both you and your baby through all the excitement. Knowing what to expect will help you get ready for the months ahead.

First Trimester Changes in Your Body

In the first trimester, a woman’s body goes through many changes. The body releases hormones that affect almost every single organ in the body. The first sign you may be pregnant is missing a period. As the first few weeks pass, some women experience the following:

  • Bleeding
  • Breast tenderness
  • Constipation and feeling bloated
  • Fatigue and tiredness
  • Food likes and dislikes/ Morning sickness
  • Heartburn
  • Weight gain

Each of the above will be discussed in more detail.

You may need to rest more or eat smaller meals during this time. Some women, however, don’t experience any of these symptoms at all – that is because pregnancy is different for every woman. Some women glow with good health during those first 3 months; others feel absolutely miserable.


Early in the pregnancy, light spotting may be a sign that the fertilized embryo has implanted in your uterus. About 25% of pregnant women have slight bleeding during their first trimester. But if you have severe bleeding, cramping, or sharp pain in your belly, call your Midwife. These could be signs of a miscarriage or ectopic pregnancy (a pregnancy in which the embryo implants outside of the uterus).

Breast tenderness

Sore breasts are one of the earliest signs of pregnancy. They’re triggered by hormonal changes, which are getting your milk ducts ready to feed your baby. Your breasts will probably be sore throughout the first trimester. Going up a bra size (or more) and wearing a support bra can make you feel more comfortable. You probably won’t go back to your regular bra size until after your baby is finished nursing.


During pregnancy, high levels of the hormone progesterone slow down the muscle contractions that normally move food through your system. Add to that the extra iron you’re getting from your prenatal vitamin, and the result is uncomfortable constipation and gas that can keep you feeling bloated throughout your pregnancy. Eat more fiber and drink extra fluids to keep things moving more smoothly. Physical activity can also help.


It’s normal to see a thin, milky white discharge (called leukorrhea) early in your pregnancy. You can wear a panty liner if it makes you feel more comfortable, but don’t use a tampon because it could put germs into your vagina. If the discharge smells really bad, if it’s green or yellow, or if there’s a lot of clear discharge, call your Midwife.

Fatigue. Your body is working hard to support a growing baby. That means you’ll get tired more easily than usual. Take naps or rest when you need to during the day. Make sure you’re getting enough iron. Too little can lead to anemia, which can make you even more tired.

Food likes and dislikes. You might not have severe cravings or dislikes for certain foods, but your tastes can change while you’re pregnant. More than 60% of pregnant women have food cravings. More than half have foods they really don’t like. Giving in to cravings from time to time is OK, so long as you eat healthy, low-calorie foods most of the time. The exception is pica – a craving for nonfoods like clay, dirt, and laundry starch, which can be dangerous for you and your baby. If you experience this kind of craving, report it to your Midwife right away.

Peeing a lot

Your baby is still pretty small, but your uterus is growing and it’s putting pressure on your bladder. As a result, you may feel like you have to go to the bathroom all the time. Don’t stop drinking fluids – your body needs them – but do cut down on caffeine, especially before bedtime. When nature calls, answer it as soon as you can. Don’t hold it in.

Heartburn. During pregnancy, your body produces more of the hormone progesterone. It relaxes smooth muscles, like the ring of muscle in your lower esophagus, the tube that connects your mouth and stomach. These muscles normally keep food and acids down in your stomach. When they loosen up, you can get acid reflux, otherwise known as heartburn. Please see our previous article on Morning Sickness

Weight gain

Pregnancy is one of the few times in a woman’s life when weight gain is considered a good thing, but don’t overdo it. During the first trimester, you should gain about 1.5  to 3 kilograms (your Midwife may suggest you adjust your weight gain up or down if you started your pregnancy underweight or overweight). Although you’re carrying an extra person, you really aren’t eating for two. You only need about an extra 150 calories a day during the first trimester. Get those calories the healthy way, by adding extra fruits and vegetables, milk, whole-grain bread, and lean meat to your diet.

What happens to the fetus during the first trimester?

The first day of your pregnancy is also the first day of your last menstrual period. At about 10 to 14 days after, an egg is released, combines with a sperm, and conception occurs. A baby develops rapidly during the first trimester. The fetus begins to develop a brain and spinal cord, and the organs begin to form. The baby’s heart will also begin to beat during the first trimester. Arms and legs begin to bud in the first few weeks, and by the end of eight weeks, fingers and toes start to form. By the end of the first trimester, the baby’s sex organs have formed.

What can be expected during my consultation at the Midwife?

When you first learn you are pregnant, make an appointment with your Midwife to begin caring for the developing baby. If you are not already on prenatal vitamins, start them immediately. Ideally, women take folic acid (in prenatal vitamins) for a year before the pregnancy. Women normally see their Midwife once a month during the first trimester and we recommend that our patients start seeing us by week 8, at which time an ultrasound would also be done.

During your first visit, the Midwife will take a full health history and perform a full physical and pelvic exam. The Midwife may also request you to do an ultrasound to determine how many weeks gestation you are.

She will also take your blood pressure, test for sexually transmitted infections, HIV, and hepatitis and other regulated blood tests, estimate your date of delivery or “due date,” which is around 266 days from the first day of your last period, screen for risk factors like anemia, and check your weight.

At around 12 weeks, the In-house Sonographer will perform a test called a nuchal translucency (NT) scan. The test uses an ultrasound to measure the baby’s head and thickness of the baby’s neck. The measurements can help determine the chance that your baby will be born with a genetic disorder known as Down syndrome.

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