For conception (fertilization), a live sperm must unite with an ovum in a fallopian tube with normally functioning epithelium. Conception occurs just after ovulation, about 14 days after a menstrual period. At ovulation, cervical mucus becomes less viscid, facilitating rapid movement of sperm to the ovum, usually near the fimbriated end of the tube. Sperm may remain alive in the vagina for about 3 days after intercourse.
The fertilized egg (zygote) divides repeatedly as it travels to the implantation site in the endometrium (usually near the fundus). By the time of implantation, the zygote has become a layer of cells around a cavity, called a blastocyst. The blastocyst wall is 1 cell thick except for the embryonic pole, which is 3 or 4 cells thick. The embryonic pole, which becomes the embryo, implants first. About 6 days after fertilization, the blastocyst implants in the uterine lining.
Within 1 or 2 days of implantation, a layer of cells (trophoblast cells) develops around the blastocyst. The progenitor villous trophoblast cell, the stem cell of the placenta, develops along 2 cell lines:
An inner layer (amnion) and outer layer (chorion) of membranes develop from the trophoblast; these membranes form the amniotic sac, which contains the conceptus (term used for derivatives of the zygote at any stage—see figure Placenta and embryo at about 11 4/7 weeks gestation). When the sac is formed and the blastocyst cavity closes (by about 10 days), the conceptus is considered an embryo. The amniotic sac fills with fluid and expands with the growing embryo, filling the endometrial cavity by about 12 weeks after conception; then, the amniotic sac is the only cavity remaining in the uterus.
Trophoblast cells develop into cells that form the placenta. The extravillous trophoblast forms villi, which penetrate the uterus. The syncytiotrophoblast covers the villi. The syncytiotrophoblast synthesizes trophic hormones and provides arterial and venous exchange between the circulation of the conceptus and that of the mother.
The placenta is fully formed by week 18 to 20 but continues to grow, weighing about 500 g by term.
Around day 10, 3 germ layers (ectoderm, mesoderm, endoderm) are usually distinct in the embryo. Then the primitive streak, which becomes the neural tube, begins to develop.
Around day 16, the cephalad portion of the mesoderm thickens, forming a central channel that develops into the heart and great vessels. The heart begins to pump plasma around day 20, and on the next day, fetal red blood cells (RBCs), which are immature and nucleated, appear. Fetal RBCs are soon replaced by mature RBCs, and blood vessels develop throughout the embryo. Eventually, the umbilical artery and vein develop, connecting the embryonic vessels with the placenta.
Most organs form between 21 and 57 days after fertilization (between 5 and 10 weeks gestation); however, the central nervous system continues to develop throughout pregnancy. Susceptibility to malformations induced by teratogens is highest when organs are forming.