By the end of the first month, which structure begins to form?

• A. Placenta begins to form
• B. Heart begins to beat
• C. Limb buds appear
• D. Neural tube closes

Answer: (A)

Early development shows that the placenta starts forming very soon after implantation. Once the blastocyst implants in the uterine lining, trophoblast cells differentiate and start creating the early placental tissues (cytotrophoblast and syncytiotrophoblast) and initial placental villi. This sets up the beginnings of the maternal–fetal interface, so by the end of the first month you can see placental formation underway.

The heart’s beating is a key milestone that occurs around the third week, which is still within the first month, but it’s a physiological function rather than a structure beginning to form. Limb buds appear a bit later, toward the end of the first month or very early in the second month, as part of limb development. The neural tube closes around the end of the fourth week as well. So the placenta is the earliest structure starting to form in this time frame.