![]() Heart development in the embryo begins at 2 to 4 weeks and is complete by the end of the embryonic stage. Implantation occurs when the cellular walls of the blastocyte implants itself in the endometrium, usually 7 to 9 days after fertilization. If the ovum is fertilized by a spermatozoon carrying a Y chromosome, a male zygote is formed. ![]() 1 Spermatozoa (or their fragments) remain in the vagina for 72 hours after sexual intercourse. The union of a male and a female gamete produces a zygote, which divides into the fertilized ovum. From the 8th week of gestation through delivery, the developing cells are known as a fetus. The corpus luteum secretes large quantities of progesterone. The chorion is the outermost extraembryonic membrane that gives rise to the placenta. Implantation in the uterus occurs 6 to 10 days after ovum fertilization. Organogenesis occurs during the first trimester of pregnancy, specifically, days 14 to 56 of gestation. ![]() Fertilization produces a total of 46 chromosomes, including an XY combination (male) or an XX combination (female). Nursing Bullets: Maternity and Newborn Nursing Reviewer Basic Concepts The male sperm contributes an X or a Y chromosome the female ovum contributes an X chromosome.
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