When a doctor is able to determine the sex of the fetus during an ultrasound, which stage of development is the fetus in?
a. third trimester
b. blastocyst formation
c. first trimester
d. second trimester

In the question,'when a doctor is able to determine the sex of the foetus, during an ultrasound scan, what stage of development is the foetus in, the correction option is D, that is, the second trimester. Human pregnancy usually lasts for an average of nine months. The nine months are divided into three parts of three months each called trimester. The first trimester is made up of the first week of the pregnancy to the twelfth week. During this period, the foetus is just developing and can not be picked up easily by an ultrasound scan, but by the second trimester, which is week thirteen to week twenty four, the foetus would have develop enough to be picked up by the ultrasound scan. At this stage, the doctor can easily determine the sex of the foetus whether it is a boy or a girl.