Background Low birth weight is a common problem in neonates.The ponderal index (PI) is a simple anthropometric measurement that is used to diagnose impaired fetal growth. PI = Weight (GM)/Length (CM) 3 × 100.

The value of ponderal index < 2.0 between 29 and 37 weeks and <2.25 beyond 37 weeks are indicative of intrauterine fetal malnutrition, and the babies are classified as disproportionate IUGR. Low ponderal index or disproportionate Intra uterine growth retardation (IUGR) has higher neonatal morbidity than their proportionate counterparts.

Objectives To determine the ponderal index of the low birth weight babies and to classify them accordingly.

Methods A descriptive cross sectional study which was carried out in Paediatrics Department of Kathmandu Medical College Teaching Hospital from July 2020 to September 2020. Ethical clearance was taken from the Institutional Review Committee.

The study population consisted of low birth weight neonates on any period of gestation. Informed written consent was taken from the parents. Babies with congenital malformations were excluded.

The sample size was 212 neonates. The eligible babies were examined after birth. Birth weight and the length were taken by standard weighing machine and infantometer. For perinatal factors, interview of parents were taken using the designed proforma.

Results A total of 210 low birth weight babies fulfilling the inclusion criteria were enrolled in this study.105 (49.5%) mothers were primiparous. 116 (54.7%) babies were born vaginally. 111 (52.4%) babies were males.

A total of 111 (52.3%) babies were premature. The mean birth weight of the babies was 2052±352 gms (1050–2490 gms) and the mean gestational age was 35±2 weeks (28–42 weeks).

Out of 212 babies, ninety-nine babies (46.7%) had a low ponderal index (<2.2 in term babies, <2.0 in preterms) and by definition had disproportionate IUGR, out of which 60 (59.4%) babies were term and 39 (35.1%) were preterms.

170 babies (80.2%) had no postnatal complications, 15 babies had neonatal sepsis, 10 babies had respiratory distress syndrome, 6 babies had hypoxic-ischemic encephalopathy (4 babies stage I HIE, 2 babies Stage II HIE), 8 babies had transient tachypnea of newborn, 3 babies had hypoglycemia and 2 babies had polycythemia.

Conclusions Ponderal index is a simple and effective tool to identify wasting. In this study, nearly 46.7% of the low birth weight babies were disproportionate at birth.