Personality at Birth What factors determine the personality of a newborn baby?
Do all newborns exhibit identical behaviour?  If not, are genetics responsible for the differences?
References consulted so far:

Is Personality More Nature or More Nurture? Behavioural and Molecular Genetics

Are Personality Traits Caused by Genes or Environment?

Do Genes Influence Personality?
The nature vs nurture debate is long standing. Nurture presumably plays little or no role for the majority of newborns (see prenatal discussion below), therefore nature (genetics) would be the only influencing factor. Subsequent to birth, nurture presumably plays an increasingly greater role as time progresses.  Some (but not all) twin studies demonstrate some impact of genetics on behaviour (Johnson, 2002; Plomin 1976).
The behaviour of newborns are generally referred to as being instinctive or innate and may include goal-directed behaviours such as suckling, looking, smelling and imprinting. Researchers such as Lorenz and Sluckin are foundational on early life behaviour research.  I also contributed to an article on newborn instinctual behaviours.
The impact of prenatal stressors on the newborn have been considered in relation to maternal mental health (Erickson, 2017), alcohol abuse (Lange, 2017; McLachlan, 2019), substance abuse (Forray, 2016) and gestational diabetes (Schmidt, 2019):

Mother’s mental health: The findings were that there was no support for relationships between prenatal mental health indicators and infant temperament in multiple studies.

Fetal Alcohol Spectrum Disorder: The reported prevalence is less than 1% of births, however, the implications on the newborn are profound with as many as 30% of adults in correctional facilities suffering Fetal Alcohol Spectrum Disorder.

Substance abuse: Whilst birth and health problems have been widely investigated, little could be found on the implications on personality of the newborn.

Diabetes: Whilst birth and health problems have been widely investigated, little could be found on the implications on personality of the newborn.


It is likely that I've not identified all relevant studies in relation to prenatal stressors affecting the personality and would welcome the advice of others.
References
Johnson, W., Krueger, R. F., Bouchard, T. J., &amp; McGue, M. (2002). The personalities of twins: Just ordinary folks. Twin Research and Human Genetics, 5(2), 125-131. https://doi.org/10.1375/twin.5.2.125
Plomin, R., Willerman, L. &amp; Loehlin, J.C. Resemblance in appearance and the equal environments assumption in twin studies of personality traits. Behav Genet 6, 43–52 (1976). https://doi.org/10.1007/BF01065677
Sluckin, W. (2017). Imprinting and early learning. Transaction Publishers.
Sluckin, W., Herbert, M., &amp; Sluckin, A. (1983). Maternal bonding. Blackwell.
Lorenz, K. (2002). King Solomon's ring: new light on animal ways. Psychology Press.
Mobbs, E. J., Mobbs, G. A., &amp; Mobbs, A. E. (2016). Imprinting, latchment and displacement: a mini review of early instinctual behaviour in newborn infants influencing breastfeeding success. Acta Paediatrica, 105(1), 24-30. https://doi.org/10.1111/apa.13034
Erickson, N. L., Gartstein, M. A., &amp; Dotson, J. A. W. (2017). Review of prenatal maternal mental health and the development of infant temperament. Journal of Obstetric, Gynecologic &amp; Neonatal Nursing. https://doi.org/10.1016/j.jogn.2017.03.008
Forray A. (2016). Substance use during pregnancy. F1000Research, 5, F1000 Faculty Rev-887. https://doi.org/10.12688/f1000research.7645.1
Schmidt, C.B., Voorhorst, I., van de Gaar, V.H.W. et al. Diabetes distress is associated with adverse pregnancy outcomes in women with gestational diabetes: a prospective cohort study. BMC Pregnancy Childbirth 19, 223 (2019). https://doi.org/10.1186/s12884-019-2376-6
Lange, S., Probst, C., Gmel, G., Rehm, J., Burd, L., &amp; Popova, S. (2017). Global Prevalence of Fetal Alcohol Spectrum Disorder Among Children and Youth: A Systematic Review and Meta-analysis. JAMA pediatrics, 171(10), 948–956. https://doi.org/10.1001/jamapediatrics.2017.1919
McLachlan, K., McNeil, A., Pei, J. et al. Prevalence and characteristics of adults with fetal alcohol spectrum disorder in corrections: a Canadian case ascertainment study. BMC Public Health 19, 43 (2019). https://doi.org/10.1186/s12889-018-6292-x