What to Look For in Ultrasound Images of Your Growing Baby Boy

For most pregnant women, finding out the sex of their fetus on ultrasound is more of a curiosity than a necessity. But for some pregnant women learning the sex of their fetus is very important, especially if they are considering invasive testing for a male sex-linked disorder like Duchenne muscular dystrophy or hemophilia.

During your pregnancy, you will likely have at least one or two routine ultrasounds. You may have more if your doctor thinks they are indicated. Except in certain unfortunate circumstances, prenatal ultrasounds are not done specifically just to check on the fetal sex.


Everything to Know About Ultrasounds

Remember, a healthy baby is your ultimate goal and the real reason for prenatal ultrasound is to help make sure you achieve that goal. Here is what that ultrasound may look like at different stages when you're carrying a boy.

Early Ultrasound: It's All About the Angle

Having an ultrasound between 11 to 14 weeks is common. An ultrasound at this gestational age will help confirm your due date, and it is part of the early risk assessment testing (ERA) for a chromosomal abnormality, such as Down syndrome.

The ultrasound technician will measure the fold on the back of the fetal neck. Both a boy and a girl fetus have this fluid-filled space. If the thickness of the nuchal translucency is more than 3.5 mm it is associated with an increased risk of Down syndrome.

At this gestational age, you can't distinguish between the genders on ultrasound. The part of the fetus that is developing into either girl parts or boy parts is called the genital tubercle. It turns out that the way the genital tubercle forms into the penis of a boy fetus is different from how it forms into the clitoris of a girl fetus.

Findings on 11- to 14-Week Ultrasound That Suggest a Boy

By looking at the angle of the genital tubercle, it is possible to distinguish between a boy and a girl fetus. But remember, this is usually not part of the routine ultrasound at this gestation age. The ultrasound technician needs to look at certain additional views of the developing fetus in order to determine this angle. Not all technicians feel comfortable with making these measurements and confirming a boy fetus.

Setting The Stage: What Needs to Happen to Make a Boy

A lot has happened by the time your boy fetus reaches 16 weeks. Even before the genital tubercle develops into boy parts, changes occur to set up the development of these recognizable boy parts—namely the penis, scrotum, and testicles.

The first step in becoming a boy starts with the fertilization of your egg with a sperm containing a Y chromosome. The SRY gene or the sex-determining region on the Y chromosome determines that testicles will develop.

By six weeks, the early testicle cells release a substance that prevents female genital development. By nine weeks, testosterone is produced. Testosterone is the major sex hormone in men and it is necessary for the continued fetal development of a boy's sex organs. 

By 14 to 16 weeks, under the influence of testosterone, the genitals of a boy fetus begin to be recognizable.

Confirming Your Dates

It is not routine to have an ultrasound at 16 weeks, however, sometimes you are late to prenatal care or your dates may be off and you end up being farther along than you thought. In these situations, you may actually be sixteen weeks (or more) at the time of your first ultrasound.

At this point, it is too late to do the early risk assessment testing. But the technician will take measurements of your fetus that will help establish or confirm your due date.

By 16 weeks, all of the parts of your boy fetus are formed. Now fetal development is focused on growth.

Preterm Labor Risk Assessment: Cervix Length

Another reason you may have an ultrasound at 16 weeks is to check the length of your cervix. This is typically done if you delivered a preterm baby (before 37 weeks) in a prior pregnancy. Unlike the typical prenatal ultrasound, this is a transvaginal ultrasound and is done only to measure your cervix.

Although not well understood, research suggests that a male fetus is associated with an increased risk of preterm labor. So, if you have a history of preterm delivery and you are pregnant with a boy baby, be sure to discuss cervical length checks with your doctor.

By 16 weeks, you can start to see a more well-defined penis and scrotum.

Anatomy Scan: The Big Reveal

It is recommended that you have an ultrasound between 18 to 20 weeks. The purpose of this ultrasound is to be sure that your fetus is developing normally. This ultrasound is often called an anatomy scan.

It is possible you may be scheduled for a more comprehensive and detailed ultrasound. This special ultrasound is often called a level 2 ultrasound and will likely be recommended If you meet certain criteria including (but certainly not limited to):

  • Age > 35
  • Diabetes
  • Early exposure to certain drugs or medications
  • Are having twins
  • Have a family history of a congenital malformation like a heart defect

And yes, this is the time that ultrasound can more reliably confirm that you are having a boy (if he cooperates).

Checking on All the Fetal Anatomy

The anatomy scan is about much more than finding a penis.

The ultrasonographer will look systematically at your fetus to be sure that all is developing normally. This is the time when congenital malformations are detected. Some of the congenital malformations found on routine ultrasound include:

  • Congenital heart defects
  • Club foot
  • Cleft lip/palate

The anatomy scan can also detect non-specific structural changes that can be associated with Down syndrome, especially if more than one of these findings are noted on the anatomy scan. These soft markers of Down syndrome include:

  • Echogenic bowel
  • Echogenic intracardiac focus
  • Short femur
  • Pyelectasis

Camera Shy?

Besides hearing that your fetus is developing normally, learning the sex of your fetus is arguably the most anticipated part of the anatomy scan.

By 18 weeks, the penis and scrotum are well developed and usually easily identified. The one obstacle to seeing if you are having a boy is the position of the fetus in your uterus. Sometimes the ultrasonographer is not able to get a good enough shot of the penis and scrotum to definitively say, "It's a boy!"

Mid-Trimester Follow-Up

Having an ultrasound at 21 weeks is not part of routine prenatal care.

However, there are certain instances where you may find yourself having an ultrasound at this gestational age. Common reasons why you may have an ultrasound at 21 weeks include:

  • Follow-up cervical length checks
  • Follow up to your anatomy scan if all structures were not adequately seen at 18 to 20 weeks
  • If you are really late to prenatal care, this may be your first dating ultrasound.

If you are having this unscheduled check, it does give you a chance to peek at those boy parts again. And if you are lucky, they are obvious enough to catch in a profile shot.

Monitoring a Complicated Pregnancy

Any ultrasound after a completed anatomy scan is not part of routine prenatal care.

Usually, it means there is something complicating your pregnancy that needs to be followed. Maybe you have diabetes or high blood pressure, or maybe something was noted on your anatomy scan that needs to be reevaluated. The top three reasons that you may need ultrasounds during your third trimester include a need to check on:

  • Fetal growth
  • Placenta location
  • Amniotic fluid volume

Managing your way through a complicated pregnancy with all of the extra testing and appointments can be stressful. But hopefully getting another look at your baby boy will brighten your day.

Descent of the Testes

The descent of the testes into the scrotum is a very important part of the normal development of your boy fetus.

It is a complicated process that begins when your boy fetus is only 10 weeks old. Between 10 to 15 weeks, the testes begin to move out of the fetal abdomen and into the area where the boy sex organs are forming. They stay in the fetal pelvis until about 26 to 35 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 percent of boy fetuses.

Testicular descent is absolutely dependent on proper testosterone production by a male fetus. If testosterone production is inadequate or disturbed in any way, a condition called cryptorchidism can result.

About 3.4 percent of newborn boys delivered at term will have cryptorchidism. However, there is significant and disturbing evidence that this anomaly is on the rise thanks to in utero exposure to endocrine-disrupting chemicals.

Checking In: Antenatal Testing in the Third Trimester

Again, although not part of routine prenatal care, having an ultrasound in the few weeks before your due date is not uncommon. An ultrasound ordered at this point in pregnancy is often part of the biophysical profile—a specific test used to check for fetal wellbeing. 

It is very likely that you will get a glimpse of what are now very obvious boy parts during this type of ultrasound. This image clearly shows a penis and scrotum, no interpretation necessary.

A Word From Verywell

Ultrasound photos are an exciting part of the pregnancy journey, but with an uncomplicated pregnancy, you may only get a glimpse or two. Still, it's helpful to understand the process of your baby boy's development so you know what to expect from the photos or just when visualizing the general stages of his growth.

Remember: Getting extra testing because you have a high-risk pregnancy is to help ensure that your nine-month journey goes well. While this can undoubtedly be stressful, these bonus sneak peeks at your baby boy on ultrasound can be a reminder (although probably an unnecessary one) of why these checks are all worthwhile.

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