Final answer:
In gestation, a regular heartbeat and blood flow through vessels in a fetus begin around day 21 or 22 post-fertilization, which is about three weeks after fertilization.
Step-by-step explanation:
The first point in gestation when a fetus has a regular heartbeat and blood begins to flow through vessels is around day 21 or 22 post-fertilization, which corresponds to about three weeks after fertilization. At this stage, the heart as the first functional organ starts to beat and establishes blood circulation essential for delivering nutrients and oxygen and for removing wastes.
Development of blood cells and vessel production in structures outside the embryo begin about 15 to 16 days following fertilization, with the heart itself beginning to beat approximately two days later, at the start of the fourth week of embryonic life.