Principles of Embryology

Pregnancy is usually divided into three trimesters of 13 weeks. However, it can be divided more precisely into three phases: implantation and predifferentiation, organogenesis (or embryogenesis), and fetogenesis. Table 47-2 describes these phases and the effects that drugs could have if taken during these phases. The risk of birth defects is higher during organogenesis.

Teratogens

A teratogen is an exogenous agent that can modify normal embryonic or fetal development. Teratogenicity can manifest as structural anomalies, a functional deficit, cancer, growth retardation, and death (spontaneous abortion, stillbirth).

Table 47-2 Phases of Embryonic and Fetal Development phii+of

PtwIWIMnt iT+gco' Prqamiy

What Hjppeni During TWs Phase of Pevtlfljirntrti

POWrlTlil T«ritr>grnl<

Effect

Knpldnralion and

OrgaiicgeiKrHS

fiiogonciii

(fiiifliin from day 14 until (hi 9th yiWk alter -conception

■AlW IIV? orgMtogenesli md urUil birth

Ait-otnonepefiod

Wry lirtkf Lr*v|.*.i1»twtitftlhii6liit«yil Jrtd (hi nVXhti'i blood

Cells are plurlpcflent, capacity lo lepaii a damage remains

CtHk jr? ft jgile jt it it. moihent. tf 100 rnjny jrO Killed. a mfe ■uti.'Kjf1 occur beiae the p<egnancy It detected Ogans a:e Immed; moa criticai period for sfructuia) anomalies Qjc^Ji^ JrO fcrrTied .11 diiffitflfll tinhCy |)OriOd Ci v-'lililivily

Tor a potential teratogen could be dilfonen: Jor each organ Furfer to F igurc 47-1 (or the time hame of organ formation tiw fetus growi and OiQjns begin to furetenitjj. tiJneys ae formed diflncj The organogenesis. hut qlofnerukv nitration begim during fetogenesb) Active ceil (tiCMfl h. jxoiifeiJdc*> and mrguiioa imh tul»ly in the £tfi

Spontaneott aborlionor ttfiitJitlAgO Even If sicpped Lk'irvj this pciiod, prolonged half' IrtL'drutJi CCxAJCJuie organogenesis piohlmis Major or minor slruc tural irtonru*«

F i 'till growth Murdjtbfl Functional deficit (jet), tenal Insufficiency, pultnonary hypeiienjjon, n^utolccjlc impaiimcnti

- CntfTpoC penod Cwfcl--fc"

- r 5-Jl Cflnat I»1S ■----//■-

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FIGURE 47-1. Embryonic development. The horizontal bars represent potential sensitivity to teratogens. The colored areas represent the more critical times. (Reprinted, with permission, from Moore KL. The Developing Human. New York: Elsevier; p 96; copyright 1974.)

FIGURE 47-1. Embryonic development. The horizontal bars represent potential sensitivity to teratogens. The colored areas represent the more critical times. (Reprinted, with permission, from Moore KL. The Developing Human. New York: Elsevier; p 96; copyright 1974.)

Criteria have been proposed to determine if a causal relationship between congenital anomalies and a medication is plausible (teratogenic effect).3 Four essential criteria have been identified. First, exposure to the medication during the critical period of development for a defect is essential. In addition, two out of three of the following criteria must be present: pattern of anomalies or syndrome AND consistent effect in at least two epidemiological studies OR rare anomaly associated to a rare exposure.

Other criteria yield a stronger causal effect but are not essential:

• Same effects observed in animal studies

• Biological plausibility based on pharmacologic effect

• Higher incidence of the anomaly in the population with the use of the medication followed by a diminished incidence of the anomaly in the absence of the medication.

Using these criteria, there are approximately 30 medications established to be teratogens. They are shown in Table 47-3. Some medications have been associated with a higher risk of anomalies; however, they do not have all the criteria to be classified as teratogens.

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