Expert: Florence M Rollwagen - 1/31/2009

Question
during which trimester of fetal development can sex be determined?

Answer
Hi Tyler:  Thanks for your question.

To answer fully, we have to determine which method we are using.  For ultrasound, the most common method, gender can be determined at about 20 weeks.  Some new equipment can detect gender at 11-15 weeks.  It's important to remember, however, that just because the operator does not detect the penis, that does not mean the child is a girl.  Fetal position, the amount of fluid, and thickness of the uterine wall can all make this kind of determination problematic.

More invasive techniques, such as detecting Y chromosome DNA in the mother's circulation, can be carried out as early as the first trimester, but these techniques are more experimental.  They require a needle stick to collect blood.

Another invasive technique is chorionic villus sampling, which is a lot more invasive and dangerous.  A sample of the uterine wall is taken by ultrasound guided needle, and again, looks for fetal Y chromosome material.  It can be carried out at about 11-15 weeks.

Here's a reference:
http://www.medicinenet.com/script/main/art.asp?articlekey=52367

Hope this helps!

FM Rollwagen, PhD