Human pregnancy typically lasts weeks and is divided into trimesters. A fetus with XX chromosomes is a while a fetus with XY chromosomes is . A fertilized egg is called a ___ . After it attaches to the uterus wall it is called a . Name the 2 functions of the placenta: Name 2 things that happen during the second trimester: Name 2 things that happen during the third trimester: Increased and help with understanding of safe pregnancies.