American Institute of Ultrasound in Medicine - AIUM.org
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 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.
In other news, Baby-to-Be is adding fat tissue and gaining weight. Rapid eye movements (REM) are now beginning, too.
Sometime this week, your baby-to-be will reach the 1-pound mark. Even though it's best for her to spend as much time in utero as possible, if she were born at this point she would have a chance of survival, although she'd most likely face serious learning or physical disabilities as a result of her premature birth.
Terms to Know
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.
Important Information About Your Pregnancy
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Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).