Final answer:
hCG production increases significantly only after the blastocyst implants into the uterine wall, not immediately after fertilization. Implantation and hCG secretion are critical for maintaining the corpus luteum and ensuring the uterine environment is conducive for the embryo's development, detectable by home pregnancy tests a few days after implantation.
Step-by-step explanation:
hCG, or human chorionic gonadotropin, is a hormone crucial for the successful development of a pregnancy. However, hCG production does not surge immediately after fertilization. During the first 24 hours post-fertilization, the fertilized egg, now known as a zygote, undergoes cleavage as it travels towards the uterus. The zygote must reach the uterus and implant into the uterine wall within about seven days to continue its development.
Following a successful implantation, when the outer layer of the blastocyst, or trophoblast, attaches to the endometrium, the cells differentiate to form the syncytiotrophoblast. This structure is responsible for secreting hCG, which then supports the continued existence and function of the corpus luteum, a structure that produces key hormones, such as progesterone and estrogen, which are vital in maintaining the endometrial lining for the developing embryo.
Implantation is usually complete by the middle of the second week after fertilization, at which point enough hCG has been produced to be detected by at-home pregnancy tests. It is worth noting that hCG injections may be used in fertility treatments before and after egg retrieval to supplement natural progesterone production.