Ms. Hudson is pregnant with twins. Which of the following is a characteristic of fraternal twins?

Ms. Hudson is pregnant with twins. Which of the following is a characteristic of fraternal twins?

• A. A single ovum has divided into two embryos.
• B. They will be the same sex.
• C. They are more likely to occur in younger women.
• D. There are two placentas.

Answer: (D)

Fraternal twins come from two separate eggs fertilized by two separate sperm, so they are dizygotic. Because two zygotes develop, each twin typically has its own placenta, making two placentas common. This distinguishes them from identical twins, which originate from one egg that splits and usually share a placenta. Fraternal twins can be the same or different sex, so a statement that they will always be the same sex isn’t correct. And twinning is more likely with older maternal age, not younger. So the characteristic that there are two placentas best reflects fraternal twins.

Fraternal twins come from two separate eggs fertilized by two separate sperm, so they are dizygotic. Because two zygotes develop, each twin typically has its own placenta, making two placentas common. This distinguishes them from identical twins, which originate from one egg that splits and usually share a placenta. Fraternal twins can be the same or different sex, so a statement that they will always be the same sex isn’t correct. And twinning is more likely with older maternal age, not younger. So the characteristic that there are two placentas best reflects fraternal twins.