The yolk sac is a membranous sac that provides nutrients to a developing embryo. It is generally visible via ultrasound by the 4th week of gestation.

The endoderm is the deepest (most proximal) germ layer of a developing embryo. It forms several endocrine glands including the pancreas, the thyroid, and the thymus. (the adrenal glands and kidneys, however, are derived from the mesoderm). It also forms the bladder and ureters, the respiratory tract, and the epithelial linings of the GI tract and auditory system.

The nervous system is derived from the ectoderm.

Over 85% of miscarriages occur in the first trimester. Less than 10% of miscarriages occur in the second trimester, and miscarriage in the third trimester is very rare and generally considered to be stillbirth or preterm delivery that result in loss of the baby due to complications (most often respiratory or digestive underdevelopment).

The three germ layers formed during embryonic development are the endoderm, the mesoderm, and the ectoderm. The trophoderm is not a germ layer; it is the membrane that forms the wall of the blastocyst in early embryonic development.

The mesoderm forms skeletal muscle, cardiac muscle, smooth muscle, connective tissue, the pericardium (in addition to several other membranous sacs such as the peritoneum) and red blood cells. It also gives rise to cartilage and bone, the dura mater, the kidneys, and the adrenal glands.

The thyroid is derived from the endoderm.

The term for the development of specialized cell types is differentiation. This is a process by which pluripotent embryonic stem cells, all of which have the same genome, are methylated in various ways to "turn off" certain genes and leave other genes exposed for transcription. This allow cells to develop the specific features that they will require to perform their function as heart cells, neurons, etc.

The neural tube forms from the neural plate during the fourth week of embryonic development, which is often before the mother knows of the pregnancy.

A zygote is a eukaryotic cell formed by a the joining of two gametes (sperm and egg). The zygote then goes on to divide several times till it forms a small clump of cells termed a morula. The next stage is the formation of a blastocyst, which is the final stage before development into an embryo.

The ductus venosus is a small shunt that allows a portion of fetal blood to bypass the liver and flow from the umbilical vein to the vena cava. This allows more oxygenated blood to be available for the developing fetal brain.

The ductus arteriosus and foramen ovale are both fetal cardiac shunts.

Fertilization happens in the fallopian tube, most frequently in the distal two-thirds of the tube.