questions for modeling brain growth using GAM

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questions for modeling brain growth using GAM

Hi, R experts

I am new to R & GAM toolbox and would like to get inputs from you all on my models. The question I have is as follows:
I have 30 subjects with each subject being scanned from one to three times in the first year of life. The brain volume from each scan was measured. The scan time was randomly distributed from birth to 1 year.
Each subject has different gestational age ranging from 38 to 41 weeks
Each subject has chronological age from birth to 1 year old
Each subject has gender category.
Now, I want to look at how predictors, such as subject's chronological age, gestational age and gender will explain the changes in brain volume. I also want to include interactions between gender and age, gestational and chronological age. Random effects are also included in the model to account for subject variability. My model looks like the follows:

gam=gam(brainVolume~ s(age) + ti(age, gestationalAge) + gestationalAge + sex + s(age, by=sex) +  s(subjIndexF, bs="re"), method="REML", data=mydata)

Are there any obvious mistakes in the model? Any suggestions will be greatly appreciated!