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A Guide To The First Trimester Of Pregnancy

A Guide To The First Trimester Of Pregnancy

The first trimester of pregnancy is the first 13 weeks of pregnancy, with the first day being the first day of the last known menstrual period. During this time, the major organ systems of the fetus develop, which makes this one of the most crucial times in pregnancy. In this article, we go through the many changes that occur in the first trimester of pregnancy, and other questions and concerns that parents may have.

Signs and symptoms of pregnancy

There are many symptoms of pregnancy, though the primary sign of pregnancy is missing 2 or more consecutive menstrual periods. However, this may not always mean you are pregnant, as conditions like polycystic ovary syndrome can also cause it.

The signs and symptoms of pregnancy can vary from one woman to another. However, there are some common symptoms that occur in a vast majority of pregnancies:

  • Fatigue – This is due to the production of progesterone, a hormone that modulates the immune system and helps normal implantation of the fertilized egg.
  • Nausea and vomiting – While most people refer to is as “morning sickness,” 80 percent of pregnant women feel sick throughout the day and/or at night.
  • Food cravings/aversions and mood swings – May be related to hormonal changes that occur during pregnancy.
  • Increased urinary frequency – This is related to the hormone Beta-Human chorionic Gonadotropin (B-HCG), which increases pelvic blood flow.
  • Tender, swollen breasts – In preparation for eventual breastfeeding, progesterone helps in the development of milk ducts and causes the breasts or nipples to swell.
  • Vaginal bleeding – Although not always present, scanty amounts of bleeding during the first trimester may be related to implantation of the fertilized egg (implantation bleeding).

Early Signs of Pregnancy: Discharge and Other Bodily Changes

When to go for a checkup and a clinical pregnancy test

It’s best to go for a prenatal checkup as soon as you suspect a pregnancy. This is necessary to ensure that the mother and fetus can get the appropriate exams and diagnostics. These tests aim to confirm that the fertilized egg has implanted itself properly.

During the initial prenatal visit, physicians can perform other clinical tests to confirm the pregnancy, such as a sonographic evaluation of the pregnancy (transvaginal ultrasound) and blood tests.

Fetal development

First month of pregnancy

After implantation, the first week of pregnancy begins. As a result, limb buds (structures that will eventually become the arms and legs), as well as the eyes and ears, begin to form. As times goes on, the heart develops a regular rhythm (around week 4) and the brain begins to develop. Simultaneously, the tissues that will turn into the vertebrae and bones start to appear.

Second month of pregnancy

As the fetus’ brain continues its development, some electrical activity begins in the nervous system. Meanwhile, the arms and legs grow longer, and fingers and toes begin their form. Likewise, the eyes, upper lip, nose and ears become visible, while the lungs start to develop. The neck and trunk of the fetus also become more defined.

Third month of pregnancy

During this time, the fingers and toes are distinct and have nails, while the fetus’ face gains more definition. Meanwhile, the fetus can now swallow and its heartbeat becomes detectable with a machine. The kidneys also begin making urine while the bone marrow forms blood cells.

It is also at this time that the doctors can determine the fetus’ sex. In female fetuses, the ovarian follicles begin forming. While in male fetuses, the prostate appears.

High risk pregnancies

These are pre-existing conditions that can lead to complications during the first trimester of pregnancy.

  • First pregnancy after 35 years of age – Women who have their first pregnancy after 35 may be at a higher risk of pregnancy complications.
  • Pregnancy during adolescence – Pregnancy during one’s teen years may result in gestational hypertension, anemia, and preterm/early labor and delivery.
  • High blood pressure – Pregnant women with uncontrolled blood pressure prior to pregnancy are at risk of developing preeclampsia, low fetal birth weight, and maternal kidney damage.
  • Obesity and/or diabetes – Pregnant mothers who are diabetic prior to pregnancy have an association with delivering larger babies, which are more difficult to deliver and may develop insulin resistance.
  • Polycystic Ovary Syndrome (PCOS) – Patients with PCOS are at a higher risk of miscarriage, gestational diabetes, and preeclampsia.
  • Thyroid Disease – Patients with hypothyroidism may experience fetal development problems.
  • Smoking during pregnancy – This increases the risk of birth defects and Sudden Fetal Death Syndrome (SFDS).
  • Illicit drug use during pregnancy – Use of drugs such as marijuana while pregnant increases the risk of stillbirths.
  • Alcohol use during pregnancy – This places the baby at risk for Fetal Alcohol Spectrum Disorders (FASDs).


TORCH syndrome is a group of infections in newborns that can occur while the baby is still in the mother’s womb. These include infection from Toxoplasmosis, Other Agents, Rubella, Cytomegalovirus, and Herpes Simplex. Newborns who develop an infection from the aforementioned agents are at risk for developing eye abnormalities, jaundice, and hearing impairments.

Safe Sex During Pregnancy: What You Need to Know

Vaccination and healthy practices

Patients should also ask their physicians as to what vaccinations they should undergo, since not all vaccines may be given during pregnancy. Vaccinations for the flu (influenza vaccine) and tetanus, diphtheria, and pertussis (Tdap) are highly recommended. These do not affect pregnancy and protect both the fetus and mother against these infections. Vaccines such as your chickenpox, MMR, and shingles should be avoided during pregnancy.

During the first trimester, it is important to follow your obstetrician’s advice regarding lifestyle modifications and follow ups. Intake of folic acid supplements, diet, appropriate weight gain throughout pregnancy, and exercise can lower the risk of developing complications.

Key Takeaway

There are many changes in the first trimester of pregnancy. This is a very important time, as this is when the fetus begins developing. Identifying possible risk factors for pregnancy complications is helpful for both doctors and parents when formulating a plan for maintaining a healthy pregnancy.

Learn more about Being Pregnant here.


Due Date Calculator

Use this calculator to find your due date. This is just an estimate – not a guarantee! Most women, but not all, will deliver their babies within a week before or after this date range.

Cycle Length

28 days

Hello Health Group does not provide medical advice, diagnosis or treatment.


Pregnancy The Three Trimesters https://www.ucsfhealth.org/conditions/pregnancy/trimesters Accessed January 22, 2021

What are some common signs of pregnancy? https://www.nichd.nih.gov/health/topics/pregnancy/conditioninfo/signs Accessed January 22, 2021

Progesterone and Progestins https://www.hormone.org/your-health-and-hormones/glands-and-hormones-a-to-z/hormones/progesterone#:~:text=What%20Does%20Progesterone%20Do%3F,body%20to%20reject%20an%20egg. Accessed January 22, 2021

Stages of Fetal Development – First Trimester https://ldh.la.gov/index.cfm/page/986 Accessed January 22, 2021

What are some factors that make a pregnancy high risk? https://www.nichd.nih.gov/health/topics/high-risk/conditioninfo/factors Accessed January 22, 2021

Do’s and don’ts during the first trimester of pregnancy https://news.sanfordhealth.org/womens/dos-and-donts-during-first-trimester-pregnancy/ Accessed January 22, 2021

Pregnancy week by week https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/expert-answers/vaccines-during-pregnancy/faq-20057799 Accessed January 22, 2021

What infections can affect pregnancy? https://www.nichd.nih.gov/health/topics/pregnancy/conditioninfo/infections Accessed January 22, 2021

TORCH Syndrome https://rarediseases.org/rare-diseases/torch-syndrome/#:~:text=TORCH%20Syndrome%20refers%20to%20any,and%20(H)erpes%20Simplex. Accessed January 22, 2021

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Written by Kip Soliva Updated Jan 22
Fact Checked by Hello Doctor Medical Panel