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 teratogen is a substance that has been known to cause birth defects. Some teratogens include alcohol, tobacco, phenytoin, rubella virus, and tetracycline. Carcinogens are a subclass of mutagens which have been known to cause cancer-inducing mutations. Mutagens cause mutations in cellular DNA expression (which may or may not lead to cancer or any other pathological condition), and pyrogens are substance that cause fever.

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. 

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). 

While all of the hormones listed are important for development of a healthy embryo, the most important hormone for preventing early first trimester miscarriage is progesterone. This hormone is generally secreted by the corpus luteum in sufficient quantities to prevent the shedding of the endometrial lining after implantation. When progesterone is insufficient, the endometrium can be shed as in a regular menstrual period, resulting in loss of the pregnancy. In most cases this type of miscarriage can be easily prevented by supplementation with vaginal progesterone suppositories. 

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.

Defects in the closure of the neural tube, a rudimentary precursor to the nervous system, can result in several serious conditions. Of these the most common are spina bifida and anencephaly. Cleft lip and cleft palate are also common defects of neural tube development. Defects in the cloaca and archenteron would affect gastrointestinal development, while the bulbus cordis is a part of the developing cardiovascular system.

The fetal heartbeat is generally first detected at 7 weeks. This can be monitored by ultrasound and auscultated. Fetal heartbeat is an important aspect of assuring fetal health through pregnancy.

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 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.