Newborn Chihuahua puppies already have brachycephalic skulls, study finds

Newborn Chihuahua puppies enter the world with skulls that are already distinctly brachycephalic — shorter, wider and higher than those of non-brachycephalic breeds — and with a bregmatic fontanelle that is both more common and relatively wider than in other puppies, according to a prospective cross-sectional study of 50 full-term newborn puppy cadavers conducted in Finland between 2020 and 2024. The findings, published in Frontiers in Veterinary Science, also suggest that the persistent fontanelles so often seen in adult Chihuahuas at sites away from the bregma are probably not present at birth, pointing instead toward postnatal development as a possible mechanism.

The research team, led by veterinarians and a veterinary pathologist at the University of Helsinki together with a craniofacial specialist from Helsinki University Hospital, examined crania from 24 Chihuahuas and 26 non-brachycephalic puppies that had died during parturition or within six days of birth. Puppies born before 57 days of gestation, brachycephalic breeds other than Chihuahuas, and animals with visible malformations were excluded. The non-brachycephalic group included Collies, Labrador retrievers, German shepherds and several other breeds. None of the puppies were euthanized for the study, and participation by owners was voluntary with signed consent under an ethics statement from the Viikki Campus Research Ethics Committee.

The investigators combined three imaging approaches with histology. Micro-computed tomography, with voxel sizes between 40 and 57.5 micrometers, was used to search the entire calvarium for fontanelles — membrane-covered gaps at the intersections of cranial sutures where bone is completely absent. Standard CT provided craniometric measurements, and histological sections stained with hematoxylin and eosin served as the definitive test, because soft tissue and cartilage can look deceptively similar to missing bone in CT images. Two assessors evaluated the micro-CT images independently and blinded to breed, history and the other modalities, reaching consensus where they disagreed; inter-rater reliability exceeded 99 percent.

The fontanelle analysis ultimately rested on 40 puppies with complete micro-CT and histological data: 19 Chihuahuas and 21 non-brachycephalic puppies. Every Chihuahua in that analysis had at least one cranial fontanelle, compared with only 6 of the 21 non-brachycephalic puppies, a difference the authors report as highly significant, with a risk difference of 71.4 percentage points (95 percent confidence interval 47.6 to 88.8 percent). All but one puppy had a single fontanelle, located at the bregma, the junction of the frontal bones. The sole exception was a Chihuahua with three fontanelles: the bregmatic fontanelle plus two symmetrical fontanelles in the frontoparietal sutures. That puppy also had cranial proportions in the upper quarter of the Chihuahua group.

A subtler finding concerned the mastoid fontanelle site, at the back and sides of the skull, where persistent fontanelles are common in adult toy breeds. In micro-CT images, every puppy in the study showed a hypoattenuated, bone-free-looking area at this location. Histology, however, revealed thick cartilage, areas of ongoing ossification and an oblique fibrous structure resembling a suture — normal developmental anatomy rather than a true fontanelle. The authors emphasize that histological confirmation is essential at this site in newborns, since both cartilage and fibrous tissue appear dark on CT.

The craniometric measurements told a consistent story about shape. The mean cranial index — the width-to-length ratio of the neurocranium, where higher values indicate greater brachycephaly — was 86.6 in Chihuahuas versus 75.6 in non-brachycephalic puppies. Cranial height to cranial base length ratios and cranial height to length ratio were also significantly higher in Chihuahuas, each with p-values below 0.001. Notably, the newborn Chihuahua values closely matched those previously reported for adult Chihuahuas: cranial height to length ratio of 0.73 in newborns versus 0.71 in adults, and cranial index of 86.6 versus 82.24. This suggests that cranial shape may change little during growth in the breed, and that the restricted longitudinal growth and compensatory widening and heightening that define brachycephaly occur largely before birth. The cranial height to width ratio, by contrast, did not differ between the groups at birth, hinting that this adult distinction emerges later during growth.

Because puppy sizes varied, the team compared the bregmatic fontanelle’s maximum width relative to total cranial width rather than in absolute terms, using a maximum intensity projection technique to follow the fontanelle across the curved skull surface. In 44 puppies with measurable fontanelles — 20 Chihuahuas and 24 non-brachycephalic puppies — the relative width was significantly larger in Chihuahuas (p = 0.0013). In a regression analysis restricted to the 26 puppies that actually had a bregmatic fontanelle, a relatively wider fontanelle was associated with higher cranial height to cranial base length 2 ratio (p = 0.022) and higher cranial height to length ratio (p = 0.023). In other words, puppies with shorter and higher skulls — a greater degree of brachycephaly — had wider open fontanelles at the bregma, supporting a link between cranial shape and cranial ossification that appears to operate already during fetal development.

Why the bregmatic fontanelle is wider in Chihuahuas remains open. The authors discuss several possibilities consistent with their data. Selective breeding for the open fontanelle known as the molera, long considered a desirable trait in the breed and still accepted in some breeding guidelines, could influence cranial ossification before birth. The breed’s extremely small body size may also matter: in adult Chihuahuas, persistent fontanelles are larger and more numerous in lighter dogs, and smaller species tend to have relatively larger brains, which could require more room and delay mineralization of the cranial vault. The authors note that the mean minimum gestation was about two days shorter in the Chihuahuas (60.2 versus 62.0 days), but argue this small difference is unlikely to explain the marked difference in fontanelle width, particularly since all puppies were considered full term.

The most provocative implication concerns the origin of persistent fontanelles in adults. In adult Chihuahuas, roughly half of persistent fontanelles occur at locations other than the bregma, and in one study cohort 82.9 percent of adult toy-sized dogs had osseous defects of the cranium outside the bregma. Yet in this newborn cohort, fontanelles outside the bregma were almost entirely absent — only one Chihuahua had them, plus one other frontal bone defect of unknown cause. The authors argue that if these adult defects were congenital, more should have appeared in the newborns. Their indirect conclusion is that at least some persistent fontanelles outside the bregma may develop during postnatal cranial growth, possibly through bone remodeling and resorption as the rapidly growing brain — which reaches 50 to 75 percent of adult size in the first two to three months — presses against a cranial vault whose longitudinal growth is restricted. They stress that longitudinal imaging and histological studies of Chihuahua puppies at different ages would be needed to confirm this hypothesis.

The authors also caution about limitations. The final fontanelle-analysis sample fell below the planned size, and because only one Chihuahua had non-bregmatic fontanelles, the original sample size calculation no longer applied. The non-brachycephalic control group was breed-heterogeneous, some puppies shared litters — a potential source of pseudoreplication — exclusions may have introduced selection bias, and molding of cranial sutures during birth may have masked small fontanelles or slightly affected measurements, though delivery mode did not differ significantly between groups. Histological sampling itself could have missed very small fontanelles, and the study was exploratory, with no adjustment for multiple testing, so the results are framed as hypothesis-generating. Within those limits, the study establishes that the Chihuahua’s signature skull shape and its unusually wide bregmatic fontanelle are already present at birth, while the scattered bone defects of the adult skull most likely develop afterward.

Subject of Research: Cranial fontanelles and skull shape in newborn Chihuahua and non-brachycephalic puppies

Article Title: Cranial fontanelles and the association of bregmatic fontanelle width with cranial morphology in newborn Chihuahua and non-brachycephalic puppies

Article References: Rautala, E., Malkamäki, S., Jokinen, T. S., Leikola, J., Ruohoniemi, M., Junnila, J. J. T., & Kiviranta, A.-M. (2026). Cranial fontanelles and the association of bregmatic fontanelle width with cranial morphology in newborn Chihuahua and non-brachycephalic puppies. Frontiers in Veterinary Science, 13, Article 1922844. https://doi.org/10.3389/fvets.2026.1922844

Image Credits: AI Generated

DOI: 10.3389/fvets.2026.1922844

Keywords: Chihuahua, fontanelle, brachycephaly, cranial index, micro-CT, puppies, cranial ossification, bregmatic fontanelle, veterinary neurology, skull morphology, bone resorption, histology