Common bile duct dilatations are not rare in Asian children and are more frequent in females [4, 9]. Choledochal cyst is a malformation of CBD and the pathogenesis is multifactorial [10–12]. The normal range of CBD size depends on age. Siegel stated that in infancy the normal CBD size should be <2 mm, <4 mm in childhood, and <7 mm after adolescence [6].

In this study CCs had an initial size of ≥10 mm and all fusiform dilatations were <7 mm. Although all CC neonates were female, there was no sex difference in those with fusiform CBD dilatations. This is consistent with a previous study, which suggested that CC might be considered if the CBD was ≥10 mm in diameter [4]. A previous study involving neonates with CC who needed surgery revealed that the cystic form had a higher incidence than the fusiform type and that this was associated with stenotic distal CBD [11].

The most common initial symptoms of CC in infants are abdominal mass and acholic stools. Older children may present with abdominal pain, while jaundice is seen in both infants and children [12]. In this study, all the infants were asymptomatic. The biliary dilatations were found via newborn screening sonographic tests.

With the availability of routine prenatal ultrasound screening, intra-abdominal cysts may be seen in antenatal scans as early as 15 weeks of gestation [13, 14]. The differential diagnosis of a right-sided intra-abdominal cystic mass includes duodenal atresia, hydronephrosis, ovarian cyst, and CC. Biliary atresia was also noted with the CCs, although the size of these CCs was smaller than that of isolated CCs [4, 15, 16]. Redkar et al. reported 13 cases with intra-abdominal cysts antenatally that were proven to be biliary atresia or CCs after surgery. The accuracy of CC diagnosis by antenatal sonogram scanning was 15%. All cysts were >10 mm in diameter when referred [17]. In the present study, only one neonate was diagnosed prenatally among the CC cases, and the size of the cyst was 12.8 mm by postnatal ultrasound scan.

The most common forms of CCs in children are type Ia, or cystic dilatation of the CBD, and type Ic, or the cylindrical or fusiform dilatation. Type Ic CC is considered if the CBD size is >10 mm, although fusiform CBD dilatation may be seen in normal variants and secondary to bile duct obstruction [4]. According to the study by Fitzpatrick et al., the incidence of CBD dilatation (the criterion they used was CBD size ≥1.2 mm) in cholestatic neonates was 9% [3]. In addition, they reported that CC was more likely and further interventions should be needed if the size of the CBD was greater than 4 mm. In contrast, if the CBD size was <3 mm, it was unlikely to be significant. Furthermore, they suggested that if the initial CBD size was 3–4 mm with no intrahepatic duct dilatation, ultrasound follow-up every 6 months should be done and full investigations should be arranged for any increase in size or if intrahepatic duct dilatation was seen [3].

In the current study, the incidence of CBD dilatation in asymptomatic neonates was 1.17% (including both fusiform dilatations and CCs). Of these cases, 87% of the fusiform CBD dilatations were TCBDDs, and 80% of TCBDDs resolved within 6 months, respectively. Even a CBD size as large as 6.7 mm resolved spontaneously (after 46 days). The pathophysiology of TCBDDs is not clear. The cases without resolution had follow-up ultrasound between 26 and 134 days after the first scans, and none had become symptomatic on their last follow-up. These cases might have resolved with longer follow-up.

The risk factors that predict the resolution of the fusiform CBD dilatations seemed to be the size of CBD and mother's age. Those with CBD size ≥3 mm had lower resolution rate, and those whose mothers are older than 35 years took a longer time to resolve.

The family occurrence of CBD dilatation has been discussed in previous studies. As many as 34 cases (17 pairs of siblings, twins, or parent and child) have been reported [18–20]. Genetic inheritance may be related to CBD dilatation. An X-linked dominant trait or an autosomal dominant trait has been suggested to be due to the female predominance. However, in some monozygotic twins, only one of the twins has been found to have CBD dilatation, so inheritance may just be one of the factors causing CBD dilatation. In the present study, there is no family occurrence in 388 pairs of twins, one set of triplets, and 2,696 sets of siblings, all of whom are newborns. However, previous studies have reported that the age of the affected families ranges between 7 days and 65 years [18–20].

Surgery is the only definitive management for CCs and may be necessary if progressive CBD dilatation, abnormal liver function, or obstructive jaundice is noted in children with CBD size <7 mm [4]. Recent studies suggest that early surgery is safer than delayed surgery and can prevent long-term complications. Laparoscopic surgery has become increasingly available in recent years and is considered safe compared to the traditional open surgery. For asymptomatic infants, the timing of surgery should be performed within 6 months of age, when the risk of anesthesia use is minimal, as long as the patient's clinical conditions permit [13, 21–28]. Diao et al. suggested that infants with asymptomatic CCs diagnosed prenatally should receive surgery within one month of age because liver fibrosis may develop immediately after birth [29].

This is a retrospective study and some data are not recorded in the charts, including parental history of CBD dilatation. Another bug of this study is refusing follow-up ultrasound scans in some cases that were still asymptomatic. Some risk factors may become significant if there are more follow-up cases. Future studies should include the cost effects of abdominal newborn screenings, more follow-up cases, and longer follow-ups for nonresolved cases to determine if some of them will become CCs.