Though some people like to be surprised, many couples decide they would like to know the sex of their baby before they make the crucial decisions on what color to paint the nursery or whether they should put little pink dresses or tiny blue sailor suits on the baby registry. Luckily, science has enabled the modern mother to determine whether she is having a boy or a girl well in advance of her due date.
11 to 12 Weeks
Still well within the first trimester, your baby is growing and developing at a quick pace at the 11- and 12-week point. The external genitalia are very close to being completely formed at this stage in the pregnancy, so it is possible to determine the sex of your baby, though the accuracy at this point is not as high as later on in the pregnancy. In an article published in "Ultrasound in Obstretrics & Genecology," Z. Efrat et al. determined that they could accurately determine the gender of a fetus at about 98 percent accuracy at 12 weeks provided they utilized a special method called genital tubercle angle, which is not a common method for normal pregnancies. Ultrasounds are generally only used to determine sex at this early stage when there is a concern of an X-linked disorder since finding out that the baby is a girl will eliminate the need for risky, invasive genetic testing.
16 Weeks
Three weeks into the second trimester, your baby's circulatory and urinary systems have begun to work on their own, and the external genitalia has been fully formed for a few weeks now. The accuracy of the gender determination is increasing at this point, but most ultrasound techs will hesitate to give you an absolute answer just to be on the safe side. They may tell you they believe it's a boy, but suggest you only pick up blue paint chip cards instead of committing to blue nursery walls.
20 Weeks
Right in the middle of the second trimester, almost all your baby's primary systems are up and running, and your baby's main job is to get bigger from this point forward. It is around the 18 to 20 week mark that technicians will begin to give you more confident answers about the sex of your baby since both the uterus and the baby are growing larger, so the external genitalia is much easier to see. Though you may have received a verdict of boy or girl earlier, accuracy at this stage is 95 to 100 percent with the usual methods of gender determination, so you can trust the determination enough to confidently paint the nursery walls at this point.
Gender Signs
Your sonologist is looking for telltale signs in the anatomy of your baby to determine its sex. Though logically the absence of a penis might seem like a good indication that your baby is a girl, this isn't quite enough since things may just be out of view at the time. Girls will display what sonologists call a three-lines sign, which is the showing of the labia in the fetus. Boys, on the other hand, will clearly show a penis and a scrotal mass.
Reasons for Inaccuracy
Though most expectant parents will rely heavily on the determination made by their ultrasound examinations, there is still a possibility of inaccuracy even as late as 28 weeks. The reason for this is because there are several factors that go into determining anything about a fetus on an ultrasound examination. First is the size of your abdomen. Larger women may have a bit less clarity than smaller women because of the amount of tissue the sensor must move through. The size of the uterine wall will also affect the readers for this same reason. The amount of amniotic fluid present in the uterus can also be a factor in the accuracy of the readings. Finally, and perhaps most importantly, is the position and movement of the fetus. If the baby's legs are crossed, or it is otherwise presenting a part of the body other than the genital area, then determining the sex will be difficult and may have to wait until your next ultrasound appointment.
References
- "Ultrasound in Obstetrics & Gynecology": Fetal Gender Assignment by First-Trimester Ultrasound
- American Pregnancy Association: Ultrasound: Sonogram
- American Pregnancy Association: 11th Week of Pregnancy
- American Pregnancy Association: 16th Week of Pregnancy
- American Pregnancy Association: 20th Week of Pregnancy
