Fetal period

Radioactivity from " C-labeled chloroform was detected in the placenta and fetuses of mice shortly after inhalation exposure (Danielsson et al. 1986). In early gestation, accumulation of radioactivity was observed in the embryonic neural tissues, while the respiratory epithelium was more involved in chloroform metabolism in the late fetal period. 

D = Closure of the hard palate to the end of pregnancy (fetal period)... 

This study is performed in two species, a rodent (usually the rat) and a non-rodent (nearly always the rabbit). Pregnant females are fed the treated diet from the day of implantation of the embryo through to the end of gestation (i.e., through the embryonic and fetal periods). The females are euthanized shortly before they would have given birth and the femses are examined to detect any strucmral abnormalities. The dams unfortunately cannot be left to give birth namrally because they tend to eat any malformed pups before they can be examined. The fetuses are sampled and processed for detailed examinations of the soft tissues and the skeleton (see Chapters 16-21). 

Longer treatment duration (embryonic and fetal periods versus embryonic period only for pharmaceuticals). 

For candidates with a dysmorphogenic potential in FETAX, a teratogenic index (TI) was calculated where possible (Table 2). The reported mortality includes deaths during the larval period for FETAX and during the embryonic or fetal period (resorptions) for the rodent and rabbit. 

Reviewers may use a checklist similar to the one given in Table I to collect data in a concise format on one page so that differences between treatment groups and the relationships between different parameters become obvious at a single glance. Litter data endpoints can be arranged in a logic order to be considered not in isolation but in context with related endpoints in the cascade of development from ovulation to the end of the fetal period. 

Fetal period The period from the end of embryogenesis to the completion of pregnancy. 

The sensitivity of the embryo to the induction of morphological defects is increased during the period of organogenesis. This period is essentially the time of the origination and development of the organs. The critical period graph (Figure 13.7) demonstrates this point and defines the embryonic and fetal periods. 

Figure 13.7 Critical periods graph including embryonic and fetal periods. Note the increased sensitivity to teratogenic events during organogenesis.

It can assess in utero exposure. Any substance in the maternal circulation can be transferred across the placenta to the developing fetus unless it is first metabolized and eliminated (Ginsberg et al. 2004). Risk assessment of the fetal period typically relies on maternal dose. However, biomonitoring of cord blood relative to maternal blood may be important to document whether there are substantial maternal-fetal differences in exposure. Evidence on methylmercury suggests that it concentrates in the fetus (Stern and Smith 2003), whereas an evaluation of 29 pesticides suggests similar concentrations across the maternal-fetal unit (Whyatt et al. 2003). 

D.E. Hutchings, J. Gibbon and M.A. Kaufman, Maternal vitamin A excess during the early fetal period Effects on learning and development in the offspring, Develop. Psychobiol. 6(5) (1973) 445-457. 

D.E. Hutchings and J. Gaston, The effects of vitamin A excess administered during the mid-fetal period on learning and development in rat offspring, Develop. Psychobiol. 7(3) (1974) 225-233. 

Embryonic age Neural tube defects from retinoic acid, arsenic, and valproic acid (Adams, 1993 Bennett Finnell, 1998) Decreased fertility in female rats exposed to dioxin (TCDD) (Gray Ostby, 1995) Hydronephrosis with dioxin exposure during embryonic or fetal periods in rats (Couture-Haws et al., 1991 Bimbaum, 1995) ... 

Immunoreactivity towards the frog dermal peptide [D-Ala2]deltorphin I, a selective ligand for delta opioid receptors, is reportedly present in high abundance in goblet cells of the rat jejunal mucosa, from which it may be secreted into the intestinal lumen [40]. Peptide-positive epithelial cells can be detected in the late fetal period [41]. Deltorphinlike immunoreactivity is not detectable in the enteric nervous system, and its precise chemical identity and functional significance remain to be determined. 

---