Prenatal development begins with the start of pregnancy and continues until the birth of the child; the nine months of gestation is usually divided into three-month time spans or trimesters. Embryology is the study of prenatal development and is introduced in this first chapter of Unit II. Prenatal development consists of three distinct successive periods: preimplantation period, embryonic period, and fetal period (Table 3-1). The preimplantation period and the embryonic period make up the first trimester of the pregnancy, and the fetal period constitutes the last two trimesters.

TABLE 3-1

Periods of Prenatal Development

	PREIMPLANTATION PERIOD	EMBRYONIC PERIOD	FETAL PERIOD
TIME SPAN	FIRST WEEK	SECOND WEEK TO EIGHTH WEEK	THIRD TO NINTH MONTH
Structure(s)∗
Structure(s) present	Zygote to blastocyst	Blastocyst to disc to embryo	Embryo to fetus
Description of period	Fertilization and implantation	Induction, proliferation, differentiation, morphogenesis, and maturation to form structures (see Table 3-3)	Maturation of existing structures

^∗Note that structure size is not accurate or comparative.

Each of these structures of the face, neck, and oral cavity has a primordium, the earliest indication of a tissue types or an organ during prenatal development. This information about the embryological background of a structure also helps in the appreciation of any developmental considerations that may occur in these structures, especially if any disturbances occur.

These developmental disturbances can include congenital malformations, or birth defects, which are evident at birth. Most of these occur during both the preimplantation period and the embryonic period and thus involve the first trimester of the pregnancy (discussed later). Such malformations occur in 3 out of 100 cases, and are one of the leading causes of infant death. This does not include anatomical variants, which are common, such as variation in the lesser details of a bone’s shape.

Malformations can be due to genetic factors, such as chromosome abnormalities or environmental agents and factors. These environmental agents and factors can include infections, drugs, and radiation and are considered to be teratogens (Table 3-2). Females of reproductive age should avoid teratogens to protect the developing infant from possible malformations (discussed later).

Malformations in the face, neck, and oral cavity range from a serious cleft in the face or palatal region to small deficiencies of the soft palate or developing cysts underneath an otherwise intact oral mucosa. It is important for dental professionals to remember that any orofacial congenital malformations discovered when examining a patient are usually understandable and traceable to a specific time in the embryological development of the individual. Thus, the dental professional must initially understand the development of an individual’s orofacial region, including its sequential process to later understand any associated pathology present.