A few weeks ago, you probably got a sneak peek of your baby-to-be during your comprehensive ultrasound examination. If your baby was cooperative, the sonographer might have been able to get a picture of your baby's gender. You might have discovered that you are expecting a little girl — or boy. Even though you're just now learning your baby's gender, your baby's sex was determined from the very first moments of life.
When the egg and sperm come together during fertilization, each contributes a chromosome that helps form the unborn baby's gender. The mother's egg is always an X, while the father's sperm carries either an X, which means you'll be having a girl (XX), or a Y, which makes for a little boy (XY). At about week four gestation (six weeks' pregnancy), your baby-to-be starts to develop the beginnings of his or her genitalia. Called the genital tubercle, this tissue will eventually become either a penis or a clitoris at around nine weeks. If your sonographer were to give you an ultrasound exam at that early date, your baby's gender might be visible, but it would still be too difficult to determine the sex because the genitalia look very similar at this beginning stage in your baby's development. By weeks 12-14, the sonographer could probably take an educated guess, but until about week 16 it's very difficult to say with any certainty whether you're having a girl or boy.
More specific to this week, your baby is so busy that it probably feels like she's walking around in your belly. And guess what? That's exactly what she's doing, pushing her feet against the uterine wall in preparation for taking those first steps alone. At this point, your baby is about 11 inches long and weighs just over 1 pound. She is developed enough to have a chance of surviving outside of your body with intensive care. Her nostrils are unsealed, and she's capable of muscular breathing, but her lungs need a little more time to mature.
Her other organs are almost in full gear. Her pancreas can now produce insulin and makes more if she's exposed to high levels of blood sugar in your body. Her brain is still growing rapidly inside her skull, which has four plates that aren't completely closed to allow for that brain growth. Even after birth, these skull plates won't be completely joined, because the human brain triples in size during the first year of life.
Your baby's lips are more distinct, rapid eye movements (REM) are now beginning, and her eyes may even flutter open occasionally — though she won't really open them fully until the seventh month. She has more pigment in her skin this week, and more fat is accumulating beneath it.
Preterm birth: Any baby born before 37 weeks' gestation is considered premature. Premature babies often suffer from complications, both physical and developmental.
Genital tubercle: In fetal development, the earliest genital tissue that will either become a clitoris in girls or a penis in boys.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).