Human prenatal development begins when a sperm cell fertilizes an egg cell, forming a single-celled zygote with a complete set of human chromosomes. Over about 38 weeks after fertilization, this cell divides, implants in the uterus, forms major body structures, and grows into a newborn ready for life outside the womb. Understanding this timeline helps explain pregnancy care, genetic inheritance, organ formation, and why early development is especially sensitive to harmful exposures.
The first two weeks focus on rapid cell division, travel through the uterine tube, and implantation in the uterine lining. Weeks 3 to 8 are the embryonic period, when the body plan and major organs begin forming through processes such as gastrulation, neurulation, and organogenesis. From week 9 to birth, the fetus grows rapidly, organs mature, and the placenta supports exchange of oxygen, nutrients, wastes, and hormones between maternal and fetal blood without normally mixing the two blood supplies.
Key Facts
- Fertilization usually occurs in the uterine tube and produces a diploid zygote with 46 chromosomes.
- Cleavage is rapid mitotic division without major overall growth, producing a morula and then a blastocyst.
- Implantation usually begins about 6 to 7 days after fertilization when the blastocyst attaches to the endometrium.
- Embryonic period = weeks 3 to 8 after fertilization, when major organs and body axes begin to form.
- Fetal period = week 9 to birth, when growth, tissue specialization, and organ maturation dominate.
- Approximate due date from fertilization = fertilization date + 266 days, while clinical pregnancy dating often uses last menstrual period + 280 days.
Vocabulary
- Zygote
- A zygote is the single diploid cell formed when a sperm nucleus and egg nucleus unite at fertilization.
- Blastocyst
- A blastocyst is a hollow ball of cells that forms about 5 days after fertilization and contains an inner cell mass that will become the embryo.
- Embryo
- An embryo is the developing human from about week 3 through week 8 after fertilization, when the major body structures begin to form.
- Fetus
- A fetus is the developing human from week 9 after fertilization until birth, when growth and organ maturation are the main changes.
- Placenta
- The placenta is an organ that develops during pregnancy to exchange oxygen, nutrients, wastes, and hormones between the mother and fetus.
Common Mistakes to Avoid
- Confusing fertilization age with gestational age is wrong because doctors often count pregnancy from the last menstrual period, about two weeks before fertilization.
- Calling the developing human a fetus immediately after fertilization is wrong because the correct sequence is zygote, cleavage stages, blastocyst, embryo, then fetus.
- Thinking the placenta mixes maternal and fetal blood directly is wrong because exchange usually occurs across thin membranes that keep the two blood supplies mostly separate.
- Assuming all organs form during the fetal period is wrong because most major organ systems begin during the embryonic period, while the fetal period focuses mainly on growth and maturation.
Practice Questions
- 1 A blastocyst begins implantation 7 days after fertilization. About how many days remain until birth if birth occurs 266 days after fertilization?
- 2 If the embryonic period runs from week 3 through week 8 after fertilization, how many weeks long is the embryonic period?
- 3 A harmful chemical exposure occurs during week 5 after fertilization. Explain why this timing may be more likely to affect body structure formation than the same exposure during week 30.