Around week 6 of development, which major event occurs?

• A. Heart starts pumping blood
• B. Limb buds appear
• C. Gonads differentiate
• D. Brain waves detected

Answer: (A)

The main idea is the moment when the embryo develops a functioning heart to establish circulation. Around week 6, the heart tube has formed enough to beat and pump blood, creating the first true circulation that carries oxygen and nutrients to growing tissues. This heart activity supports rapid development of all organs and limbs that have already started to form, and it marks a major shift from a simple cellular mass to an actively circulating organism. Limb buds began forming earlier, around weeks 4–5, but they don’t hinge on a functioning circulation in the same way that the heart does. Gonads start differentiating a bit later, around weeks 7–9, and brain-wave activity is detected a bit later than week 6, so the heart beginning to pump is the best fit for what typically occurs around this time.