In this paper, we report the growth behaviour of long-period gratings (LPGs) in a hydrogen-loaded fibre during and immediately after 193 nm UV inscription. The LPG's peak loss follows a sinusoidal function of the amplitude of the index modulation in the fibre core, thus the peak loss increases first and then decreases with further increase of writing pulses during UV inscription. After UV inscription has stopped, the LPG continues to grow with time owing to the hydrogen diffusion, and an increase of as much as 14 dB in the LPG peak loss was measured. We offer a qualitative explanation for the growth dynamics of the LPGs immediately after 193 nm UV inscription and discuss the effects of temperature and initial grating strength on the growth behaviour. We also investigate the impact of post-UV irradiation on the LPGs after annealing. Our study shows that post-irradiation trims not only the peak loss intensity but also the peak wavelength of the LPGs. This provides an opportunity to control the resonance peaks of the LPGs.