Ultrasound Images of Your Growing Baby Boy

For many expectant parents, finding out the sex of their fetus is more of a curiosity than a necessity, but it's still an exciting milestone.

You'll likely figure out the sex at your anatomy scan around 20 weeks—though you might also learn it earlier through certain prenatal tests, such as non-invasive prenatal screening (NIPT), chorionic villus sampling (CVS), or amniocentesis.

That said, the processes that make a boy start much earlier in your gestation. If you're expecting a baby boy, here's what your ultrasound may look like at different stages

Development of a Male Fetus

Becoming a boy starts when an egg is fertilized with a sperm containing a Y chromosome. The SRY gene (or the sex-determining region on the Y chromosome) determines whether testicles will develop.

Many other steps go into the formation of a male fetus, including the following:

• Early testicle cells release a substance that prevents female genital development.
• Testosterone production starts before nine weeks.
• Around 12 weeks of gestation, under the influence of testosterone, the genitals of a boy fetus begin to be recognizable—namely the penis, scrotum, and testicles.

The descent of the testes into the scrotum contributes largely to the normal development of a boy fetus. Between eight to 15 weeks of pregnancy, the testes begin to move out of the fetal abdomen and into the area where the sex organs are forming. They stay in the fetal pelvis until about 26 weeks. Then, under the influence of testosterone, the testes begin to move down into the fetal scrotum. By 32 weeks, both testes have descended in over 95% of boy fetuses.

First Trimester Ultrasounds

You might have a first-trimester ultrasound to confirm your due date or rule out potential complications. Additionally, some pregnant people get a nuchal translucency (NT) ultrasound around 10 to 13 weeks, during which the technician will measure the space at the back of a fetus' neck to determine its risk for Down syndrome and other congenital disabilities.

At this gestational age, you can't distinguish between the sexes on an ultrasound. The part of the fetus that's forming into either girl parts or boy parts— called the genital tubercle—is still developing.

That said, it might be possible to determine sex by looking at the angle of the genital tubercle (also called nub theory). The theory says if the genital tubercle points upward, the fetus is a male. Not all technicians feel comfortable with making these measurements and confirming a boy fetus.

Anatomy Scan (18-22 Weeks)

You'll have an ultrasound between 18 to 22 weeks, called an anatomy scan (though some refer to it as a "gender ultrasound"). The technician will systematically assess your fetus to ensure that everything is developing normally, looking at the following things:

• Fetal body from head to toe
• Heart rate
• Major organs
• Size of the fetus
• Amniotic fluid levels
• Placenta position
• Sex organs

Besides hearing that your fetus is developing normally, learning the sex is arguably the most anticipated part of the anatomy scan. By 18 weeks, the penis and scrotum are well-developed and usually easily identified.

That said, on rare occasions, the technician won't be able to get a good enough shot of the penis and scrotum to definitively say, "It's a boy!"

Ultrasounds in the Second or Third Trimester

Any ultrasound done after a detailed anatomy scan isn't part of routine prenatal care. Usually, it means you have a pregnancy complication that needs monitoring, such as:

• High (polyhydramnios) or low (oligohydramnios) amniotic fluid levels
• Abnormal fetal growth
• Concerns about placenta location

During these ultrasounds, it's very likely that you'll get a glimpse of your growing baby boy. The image will clearly show a penis and scrotum, no interpretation necessary.