Determinants
Genetics
There are two genetic loci that have been strongly linked to birth weight, ADCY5 and CCNL1, as well four that show some evidence (CDKAL1, HHEX-IDE, GCK, and TCF7L2). The heritability of birth weight ranges from 25 to 40%. There is a complex relationship between a babys genes and the maternal environment that the child is developing in. Foetal genes influence how the fetus grows in utero, and the maternal genes influence how the environment affects the growing fetus. Maternal health
The health of the mother during the pregnancy can affect birth weight. A pre-existing disease or acquired disease in pregnancy is sometimes associated with decreased birth weight. For example, celiac disease confers an odds ratio of low birth weight of approximately 1.8. Certain medications (e.g. for high blood pressure or epilepsy) can put a mother at a higher risk for delivering a low birth weight baby. Women younger than 15 or older than 35 are at a higher risk to have a low-birth weight baby. Multiple births, where a mother has more than one child at one time, can also be a determinant in birth weight as each baby is likely to be outside the AGA (appropriate for gestational age). Multiple births put children at a higher rate to have low birth weight (56.6%) compared to children born in a single birth ( 6.2%). Low birth weight can also vary by maternal age. In 2008 the rate of low birth weight was the highest in babies born to women younger than 15 years old (12.4%).
Women aged 40–54 had a rate of low birth weight at 11.8 percent. The lowest rates of low birth weight happened among babies whose mothers were between the ages of 25–29 years (4.4%) and 30–34 years (7.6%). Stress
Stressful events have been demonstrated to produce significant effects on birth weight. Those mothers who have stressful events during pregnancy, especially during the first and second trimester, are at higher risk to deliver low-birth weight babies. Researchers furthered this study and found that maternal stressful events that occur prior to conception have a negative impact on birth weight as well, and can result in a higher risk for preterm and lower birth weight babies. Women who experienced abuse (physical, sexual, or emotional) during pregnancy are also at increased risk of delivering a low-birth weight baby. For example, in a study completed by Witt et al., those women who experienced a stressful event (i.e. death of close family member, infertility issues, separation from partner) prior to conception had 38% more of a chance to have a very low birth weight baby compared to those who had not experienced a stressful life event. The theory is that stress can impact a baby based on two different mechanisms: neuroendocrine pathway or immune/inflammatory pathway. Stress causes the body to produce stress hormones called glucocorticoids that can suppress the immune system., as well as raises levels of placental corticotropin-releasing hormone (CRH) which can lead to preterm labor. These findings can pose evidence for future prevention efforts for low birth weight babies.