Chorion type may significantly influence the prenatal environment of twins. This study explored the associations between chorion type and gestational age, birth weight, birth length, and the timing of emergence of the first primary tooth in two populations of twins, Australian and Dutch. Additionally, we investigated the relationship between chorion type and birth weight discordance (BWD) in order to determine whether a significant relationship existed between discordance in birth weight and discordance in the timing of emergence of the first primary tooth. The two study samples consisted of 409 Australian twin pairs and 301 Dutch twin pairs, all of European ancestry. Data were collected through a combination of questionnaires and recording charts administered to the parents and through linkage with biological databases. In the Australian sample, monozygotic monochorionic (MZMC) twins experienced the shortest mean gestation time (35 weeks), the lowest mean birth length (46 cm) and the lowest mean birth weight (2.3 kg) compared with other twin groups. For the same variables in the Dutch sample, these trends with MZMC twinning were not observed. Chorion type did not significantly affect the mean timing of emergence of the first primary tooth in either sample. Monochorionicity was found to be significantly associated with BWD in both samples, but there was a significant association between BWD in MZMC twin pairs and timing of emergence of the first primary tooth only in the Australian sample. Results from this study support previous findings that the timing of emergence of the first primary tooth is influenced strongly by genetic factors and is well protected from environmental disturbances.

The interdental and interspecific variation in the prevalence and severity of macroscopic fluorotic alterations of the permanent mandibular cheek teeth (P2–4, M1–3) was studied in 331 roe deer and 62 red deer exhibiting severe dental fluorosis. The material originated from the fluoride-polluted region of the Ore mountains and their southern foreland (Czech–German border region). Scoring of dental fluorosis was based on an ordinal measurement scale with six scores (score 0: unfluorosed, scores 1–5: increasing severity of fluorotic alterations). The observed variation in dental fluorosis both within the cheek tooth row of a species and between certain homologous teeth of roe and red deer could be related to the development of the dentition in the two species, especially the timing of tooth crown formation in relation to weaning and the subsequent period, where the animals feed upon (fluoride-contaminated) plant material. The lower prevalence (roe deer: 3%, red deer: 42%) and severity (fluorosis scores ð 2) of dental fluorosis in the M1 compared to the other permanent cheek teeth were attributed to the fact that the crown formation of this tooth takes place largely (roe deer) or to a considerable extent (red deer) prenatally and during the period of milk feeding. It is assumed that during these ontogenetic stages several mechanisms (partial placental diffusion barrier to fluoride, low fluoride content of milk, rapid clearance of fluoride from the blood resulting from a high skeletal growth rate) to some extent protect the developing teeth from increased fluoride exposure. Therefore, in both species apparently only the (late) maturation stage of M1 amelogenesis can be affected by fluoride in a way to induce visible enamel changes. The higher prevalence and severity of fluorotic lesions in the M2 of red than roe deer was related to the fact that in the former species crown formation of this tooth, like in the P2–4 and M3 of both species, takes place completely post-weaning.