By Week 3 of pregnancy, which of the following is true?

• A. The embryo begins to draw on maternal circulation.
• B. Placenta develops as early liver.
• C. The heart begins to beat.
• D. Circulation bypasses lungs.

Answer: (C)

Early embryonic development is marked by the heart’s first function. The primitive heart tube forms and begins to beat around day 22–23 after conception, which corresponds to about the third week of gestation. So, by Week 3, the heart starts beating, making this the true, defining event at this stage.

The other statements describe events that occur later or aren’t specific to this week. The placenta and its blood flow to the embryo become functionally established a bit later in the first trimester; it isn’t correct to say the placenta develops as an early liver. The fetus does rely on placental circulation for nutrients and gas exchange, but that larger-scale maternal-fetal circulation isn’t fully in place by the third week. Regarding circulation bypassing the lungs, fetal circulation does bypass the lungs, but this is a general feature of fetal life and not the specific milestone unique to Week 3; the onset of a heartbeat is the key marker at this time.