Chapter 3. Fetal Sex Identification

Welcome to the chapter on fetal sex identification. While many women ask if their baby is a girl or boy, most guidelines state that gender checks out of curiosity are not medically necessary. Some cultures have used gender information to determine whether to continue a pregnancy, which has led most countries to ban gender-specific abortions. The clinician’s responsibility is to provide accurate information while remaining sensitive to families' wishes regarding fetal sex disclosure. On the positive side, fetal bonding is often enhanced when the sex is known by the mother.

OBJECTIVES:

By the completion of this Section the participant will be able to:

Describe medical indications where sex identification in utero might be important
Discuss the ramifications of advising patients of fetal sex if it is not determined with certainty

Sex identification is appropriate when performed as part of a medically indicated examination, whether routine or point-of-care. However, it is prudent to inform all parties that even highly experienced sonographers may occasionally make errors in this assessment.

Many women currently opt for non-invasive prenatal testing in the first trimester; if so, they are already aware of the fetal sex.

Figure 1. Purpose of identifying fetal sex.

Sex identification is appropriate when performed as part of a medically indicated examination, whether routine or point-of-care. However, it is prudent to inform all parties that even highly experienced sonographers may occasionally make errors in this assessment.

Many women currently opt for non-invasive prenatal testing in the first trimester; if so, they are already aware of the fetal sex.

Medical indications for determining sex include:

Screening for x-linked genetic disease

Helping to establish zygoticity/chorionicity in twin gestations

Observing for genital abnormalities (ambiguous genitalia, for example)

Fetal sex identification depends on the equipment's imaging quality, scanner’s technique, and to some extent, body maternal habitus. But in general, it has more to do with fetal presentation and positioning.

Some countries have consensus-based practice guidelines for assessing fetal sex with ultrasound. The Society of Obstetrics and Gynecology of Canada has published the following statements":¹ R

1. Diagnostic imaging providers and other health care providers should respect parental wishes regarding disclosure of fetal sex.

2. Imaging providers should make every effort to determine fetal sex during an obstetric exam from the second trimester onward, and include this information in the ultrasound report

3. If fetal sex cannot be determined, re-examination is recommended at a later time

4. The ob ultrasound report should contain an alert about the presence of fetal sex to alert parents, giving them the option of not reading further in the report.

Image nub theory

Some seek to determine fetal sex as early as the end of the first trimester. Because female fetuses have an enlarged clitoris, prediction at this stage can be challenging. However, by examining the "nub" in a sagittal view it may sometimes be evident. If the nub points upward from the baby's axis, it is likely a penis; if it points forward, it is likely a clitoris. It is important to understand this is not a conclusive finding.

Image 1. Sagittal view of fetal sex illustrating the “nub” theory.

To identify sex, the following steps are suggested:

Find the sagittal view of the fetal spine and follow caudally on the fetus until you come to the fetal pelvis

Rotate the probe so that you can see both femurs extend in a parallel fashion

Slide the transducer slowly up and down along the baby’s torso until you see fetal bladder.

Observe between the legs to determine a penis or signs of vulva; the vulva in earlier stages may appear as 3 parallel lines

Image 2. Image of male genitalia.

Try to spot the scrotum and penis between the legs in this image.

The crotum and penis are outlined in red below.

Image 2. Image of male genitalia, annotated.

Male fetus

The male in Image 3 has a different angle; his back is upward toward the maternal skin and his scrotum and penis dangle downward

Image 3. Image of male genitalia, pointed downward.

Image 3. Image of male genitalia, pointed downward, annotated.

As the fetus grows, and with more sophisticated machines, it becomes easier to see more detail in the scrotum, and even see testicles.

Images 4,5. Images of testicles and testes of male fetus.

Female fetus

This is a transverse view of vulva in a female fetus.

Image 6. Image of female fetal vulva.

Image 6. Image of female fetal vulva, annotated in red.

Image 7. Image of female fetal vulva (indicated by arrow)

Image 7. Image of female fetal vulva (indicated by arrow), annotated in red.

Below is a 3D image of female fetal vulva (Image 8) demonstrating a more realistic view that 3D imaging can provide.

Image 8. 3D Image of fetal female vulva.

Implications of incorrect identification of sex²

Implications of an incorrect diagnosis of sex can include:

May alter certain genetic testing related to sex

May alter diagnosis of fraternal versus identical twinning

Although banned by most countries, some consider sex a reason for terminating a pregnancy

Inconvenience

Plans for baby’s environment and clothing

The Canadian Ministry of Health in 2012³ issued a policy stating that only a woman’s provider could tell her the sex of the baby, and not the sonographer. This would not apply to POCUS ultrasound, but it does demonstrate the seriousness of correctly identifying sex

SUMMARY

Determination of fetal sex is often desired by a woman and/or family members

Fetal sex determination may be important for certain clinical reasons:

--Identifying need for certain genetic testing

--Discriminating fraternal from an identical twinning

Women's Point-of-Care Ultrasound: Second and Third Trimester

Fetal Sex Identification