The Ultrasound Said It Was a Girl... But a Boy Was Born: How Is That Possible?

During pregnancy, many expectant parents eagerly await the moment they learn their baby's sex. Thanks to modern ultrasound technology, this information can often be determined before birth. However, in rare cases, the sex predicted during pregnancy does not match the baby's sex at birth.

How can this happen?

Ultrasound Is Highly Accurate—But Not Infallible

Ultrasound estimates the baby's sex by examining the external genitalia. Unlike genetic testing, it does not determine the baby's chromosomal sex. Instead, it relies on interpreting the appearance of the genital area at the time of the scan.

In most cases, particularly during the detailed anatomy scan at around 20 to 22 weeks of pregnancy, this assessment is correct. However, no ultrasound examination can guarantee 100% accuracy.

Why Can a Baby Boy Be Mistaken for a Girl?

Several factors can make sex determination more difficult.

1. The Baby's Position

If the baby has crossed legs, is facing away from the ultrasound probe, or has the genital area obscured by the hands or the umbilical cord, the penis and scrotum may not be visible.

2. The Stage of Pregnancy

Before about 14 to 15 weeks of pregnancy, the external genitalia are still developing. Early predictions are therefore more likely to be incorrect.

3. Examination Conditions

Limited amniotic fluid, a thicker maternal abdominal wall, previous abdominal scars, or suboptimal image quality can reduce the clarity of the examination.

4. A Hidden Penis

Occasionally, the penis is temporarily hidden between the baby's legs or is simply difficult to visualize. In these situations, the genital area may resemble female anatomy, leading to the prediction of a girl.

How Often Does This Happen?

When the examination is performed under good conditions by an experienced sonographer during the second trimester, the accuracy of fetal sex determination generally exceeds 95% to 99%.

Mistakes remain uncommon, but they are not impossible. This is why healthcare professionals can only provide a highly reliable prediction—not an absolute guarantee.

Could There Be Another Explanation?

In very rare cases, the difference is not due to an ultrasound error but to a difference (or variation) in sex development (DSD). Certain genetic or hormonal conditions can affect the appearance of the external genitalia.

These conditions are uncommon and require specialized assessment after birth.

How Do Parents Usually React?

For some families, the surprise quickly becomes a funny story. For others, it may bring disappointment, confusion, or the feeling of having lost the child they had imagined. These emotions are understandable and deserve compassion rather than judgment.

Fortunately, most parents rapidly form a strong bond with their baby, regardless of sex.

Can These Mistakes Be Prevented?

The risk can be reduced by:

• Performing the ultrasound at the recommended stage of pregnancy.

• Using high-quality ultrasound equipment.

• Repeating the examination if visualization is limited.

• Remembering that ultrasound-based fetal sex determination remains an estimate, even when the images appear very convincing.

  
  

In Summary

Although uncommon, it is possible for an ultrasound to predict a girl and for a baby boy to be born. In most cases, this reflects the inherent limitations of ultrasound imaging rather than a medical error.

The primary goal of every prenatal ultrasound is to assess the baby's growth, anatomy, and well-being. While learning the baby's sex is exciting, the most important outcome is always the birth of a healthy child.

Take care of yourself.

Dr. Sandra Yene Amougui