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What causes birth defects in the first trimester?

During pregnancy trimesters especially in the first months of pregnancy, there are some problems may occur to the fetus which may delay the formation of baby’s organs or sometimes may lead to the death of the fetus inside the womb (Intra-Uterine Fetal Death IUFD).

These problems are called the birth defects or the congenital abnormalities, Read on to know more information about what causes these birth defects, and in which situations especially in the first trimester.

What causes birth defects in the first trimester
What causes birth defects in the first trimester

Why is the first trimester the most dangerous for developing birth defects?

The first trimester is the most important stage of pregnancy because of the development of most of the body organs of the baby.

Also, the development of the placenta and the blood-placental barrier is incomplete which means that the harmful drugs, chemicals, and alcohol possibly pass from mother to fetus thus leading to the birth defects.

And, the pregnancy isn’t stable at this stage which means that sometimes the mild contractions of womb muscle may lead to abortion.

What causes birth defects in the first trimester?

Causes of birth defects in the first trimester are many and classed according to the source of harmful factors whether external or internal factors.

External factors that cause birth defects in the first trimester

These are the factors that come from outside the mothers body such as

  • Radiation
    • Some types of radiation are very harmful and cause deformities and birth defects to the baby, such as
      • X-ray,
      • CT radiation,
      • and radiotherapy.
  • Medications
    • A lot of medications should be stopped during pregnancy especially during the first trimester because it causes dramatic abnormalities in the baby such as
      • Aminoglycosides,
      • Non-Steroidal Anti Inflammatory Drugs,
      • and isotretinoin which is used in the treatment of severe acne.
  • Smoking and Drinking alcohol
    • Fetal alcohol syndrome (FAS) is very common in the first trimester which occurs due to drinking alcohol.
  • Certain infections
    • Sometimes the mother had infections that may be harmful to the baby such as
      • Toxoplasmosis (more harmful in the third trimester),
      • Rubella virus,
      • Chickenpox virus,
      • and Measles virus.
    • Fortunately, most people got the vaccination against these infections, but the Toxoplasmosis still give records.

Internal factors that cause birth defects in the first trimester

These are the factors that come from inside the mothers body such as

  • Medical disorders
    • Uncontrolled diabetes, Obesity, and uncontrolled hypertension may lead to abnormalities in the growing baby.
    • Also, chronic heart, liver, or kidney diseases should have the optimal care and regular follow up during pregnancy because its medications may be harmful to the fetus.
  • Family history and Genetics.
    • If there is a family history of birth defects, the mother should consult a clinical geneticist or a genetic counselor.
  • The old age of parents.
    • The old age of the mother more than 35 years old increase the chance of occurrence of birth defects.

To know more about Signs of birth defects during pregnancy, Tests that detect it, and how to avoid it, you can read This Article.


The first trimester (first three months) of pregnancy is the most dangerous of developing of birth defects.

Birth defects may be due to internal causes such as diseases, old age, or genetic factors and family history.

Also, it may be due to external causes such as smoking, alcohol, radiation, chemicals, certain medication, or infections.


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