Prenatal gender determination is a common and exciting part of the pregnancy journey. The short answer to whether a gender scan indicating a girl can be wrong is yes; while modern methods are highly accurate, no non-invasive test is completely infallible. Errors can occur due to factors ranging from the baby’s position during an imaging test to biological complexities in the mother’s blood. Understanding the differences between the primary testing methods clarifies why these occasional misidentifications happen.
Primary Methods of Gender Determination
The two main non-invasive methods used in prenatal care to determine the baby’s sex are the fetal ultrasound and Non-Invasive Prenatal Testing (NIPT). Fetal ultrasound, or sonogram, is the traditional imaging technique that uses sound waves to create a real-time visual of the fetus. This method relies on visualizing the external anatomy, specifically the genital area, to predict gender.
The optimal time for a reliable ultrasound gender check is during the second-trimester anatomy scan, typically performed between 18 and 22 weeks of gestation. At this stage, the external reproductive structures are usually developed enough to be distinguished visually. However, the prediction relies heavily on the technician capturing a clear, unobstructed view of the baby’s pelvis.
Non-Invasive Prenatal Testing (NIPT) is a highly accurate blood test performed on the mother, offering an earlier alternative. This test analyzes fragments of cell-free DNA (cfDNA) from the placenta circulating in the maternal bloodstream. The primary purpose of NIPT is to screen for common chromosomal conditions, but it also accurately identifies the baby’s genetic sex by detecting the presence or absence of the Y chromosome. Since the Y chromosome is only present in males, its absence indicates a female fetus.
Factors Causing Ultrasound Misidentification
When an ultrasound incorrectly indicates a female result, it is typically a “false female” error, meaning the fetus is genetically male but was misidentified. This error is common because the technician defaults to a female identification if they cannot definitively see the distinguishing male anatomy. The most frequent cause of this misidentification is the baby’s positioning during the scan.
If a male fetus has its legs tightly crossed, is curled up, or is facing away, the genital area may be completely obscured from the sonographer’s view. The umbilical cord or a loop of bowel can also lie between the legs, potentially hiding a developing penis. If the technician does not see clear male anatomy, the result is reported as female, which may later prove incorrect.
The timing of the scan significantly influences the accuracy of ultrasound gender determination. Scans performed earlier than 16 weeks have a lower accuracy rate because the external genitalia are still small and underdeveloped. Accuracy can be very high by 18 to 20 weeks, but earlier scans are more likely to lead to an incorrect female identification, as the developing male anatomy may not be prominent enough to be clearly visualized. Furthermore, the external genitalia of both sexes are similar in appearance in the first trimester, developing from the genital tubercle.
Comparative Accuracy of Prenatal Gender Tests
The accuracy of fetal gender determination varies substantially between methods, especially based on the timing of the test. A mid-pregnancy ultrasound (18 to 22 weeks) is highly accurate, with success rates typically ranging from 95% to 99% when the baby is positioned cooperatively. Errors are most often a false female result, where a male fetus is incorrectly identified as a girl due to difficulty visualizing the anatomy.
Non-Invasive Prenatal Testing (NIPT) provides a superior level of accuracy for gender determination, generally exceeding 98% to 99%. Since NIPT analyzes the baby’s actual chromosomes from cell-free DNA in the maternal blood, it functions as a genetic test rather than an anatomical observation. This high accuracy is available much earlier in the pregnancy, often from the tenth week onward. While NIPT is highly reliable, its rare errors stem from biological factors, unlike the visual errors of an ultrasound.
Causes of NIPT Errors
One known cause of a false female NIPT result is a low fetal fraction. This technical issue means there is not enough of the baby’s DNA in the bloodstream to detect the Y chromosome, leading to an incorrect female prediction.
A rare cause of a false male result is the vanishing twin syndrome. Here, a male twin demises early, but its DNA remains in the maternal circulation long enough to be detected, despite the surviving fetus being female.
For the highest level of certainty, especially with early or ambiguous results, confirmation from two different methods is advised, such as NIPT followed by a later, clear ultrasound image.