Summary
Proper nutrition during pregnancy is important for promoting a healthy birth weight. Studies have found that DHA supplementation during pregnancy increases the birth weight compared to control groups who did not receive DHA supplementation. Groups that received DHA supplementation also had a lower prevalence of pre-term birth (<34 weeks gestation). The increased birth weight in DHA groups is likely the result of a longer average gestation period. 
 
Introduction
Birth weight is an important indicator of an infant's health. It can be used as a  parameter for assessing newborns' development and potential health risks. Babies born with a healthy weight have a greater likelihood of thriving in the early stages of life and experience fewer health complications. Conversely, low birth weight (defined by the World Health Organization as <2.5 kg, or <5.5lbs) can pose significant health challenges for the infant.

Nutritional status of the mother during pregnancy is correlated with birth weight. Proper nutrition during pregnancy ensures that the fetus receives essential nutrients, promoting healthy weight gain. Insufficient nutrition can lead to low birth weight, increasing the risk of infections, developmental delays, and chronic illnesses.

The gestation period, i.e. the duration of the pregnancy, is an important factor that influences the birthweight. A shorter gestation period is associated with a lower birthweight, which can have negative consequences for the infants health.
Research has established a link between DHA supplementation during pregnancy, gestation period and birth weight. The beneficial effects of DHA supplementation during pregnancy are summarized below.
 
Research findings
Makrides and colleagues found that diet supplementation of 800mg DHA per day from approximately 22 weeks gestation to birth leads to an increase in birthweight compared to the control group that did not receive DHA. When corrected for gestational period and sex, there was no difference between groups. There was a reduction in the number of preterm birth at less than 34 weeks gestation in the DHA group, as well as fewer infants with low birth weight and less admissions to the neonatal intensive care. This indicates that the higher average gestational age in the DHA group is likely the reason for the higher birth weight (Makrides et al., 2010).

This finding is corroborated by Carlson and colleagues, who reported that diet supplementation with DHA during pregnancy at a dose of 600mg/day from <20 gestational weeks to birth results in an increased birth weight (Carlson et al., 2013). Here, the DHA group had fewer infants born at less than 34 weeks of gestation and shorter hospital stays for infants that were born preterm. Similar to the findings of Makrides et al. (2010), DHA supplementation was associated with a greater gestation duration.  

A follow-up study was conducted by Carlson et al. (2021) to assess a dose-response effect of DHA supplementation during pregnancy. The results indicated that supplementation of 1000mg DHA per day  during pregnancy was associated with a lower rate of early pre-term birth than supplementation of 200mg DHA per day (Carlson et al., 2021). The dose effect was the most pronounced in mothers that had a low DHA nutritional status at time of enrollment, whereas no dose effect was found in mothers that had a high DHA nutritional status at time of enrollment in the study. This indicates that DHA supplementation may be beneficial to the developing fetus by ensuring that the mother has a sufficient DHA nutritional status.
 
Conclusions
DHA is an important factor in the nutritional status of the mother during pregnancy. Studies have indicated that DHA supplementation can have a beneficial effect on the average gestation period and birth weight. The supplementation with DHA during pregnancy may decrease the chance of pre-term birth (<34 weeks gestation), by ensuring that a sufficient amount of DHA is available to the developing fetus. Mothers that supplement their diet with DHA during pregnancy have a higher average gestation period, concomitant with a higher average birth weight. The lower incidence of pre-term birth associated with DHA supplementation during pregnancy may be a health benefit of DHA supplementation.
 
Sources
Carlson, S. E., Colombo, J., Gajewski, B. J., Gustafson, K. M., Mundy, D., Yeast, J., Georgieff, M. K., Markley, L. A., Kerling, E. H., & Shaddy, D. J. (2013). DHA supplementation and pregnancy outcomes. The American Journal of Clinical Nutrition, 97(4), 808–815. https://doi.org/10.3945/ajcn.112.050021

Carlson, S. E., Gajewski, B. J., Valentine, C. J., Kerling, E. H., Weiner, C. P., Cackovic, M., Buhimschi, C. S., Rogers, L. K., Sands, S. A., Brown, A. R., Mudaranthakam, D. P., Crawford, S. A., & DeFranco, E. A. (2021). Higher dose docosahexaenoic acid supplementation during pregnancy and early preterm birth: A randomised, double-blind, adaptive-design superiority trial. eClinicalMedicine, 36. https://doi.org/10.1016/j.eclinm.2021.100905

Makrides, M., Gibson, R. A., McPhee, A. J., Yelland, L., Quinlivan, J., Ryan, P., & DOMInO Investigative Team,  and the. (2010). Effect of DHA Supplementation During Pregnancy on Maternal Depression and Neurodevelopment of Young Children: A Randomized Controlled Trial. JAMA, 304(15), 1675–1683. https://doi.org/10.1001/jama.2010.1507

Ramakrishnan, U., Stein, A. D., Parra-Cabrera, S., Wang, M., Imhoff-Kunsch, B., Juárez-Márquez, S., Rivera, J., & Martorell, R. (2010). Effects of docosahexaenoic acid supplementation during pregnancy on gestational age and size at birth: Randomized, double-blind, placebo-controlled trial in Mexico. Food and Nutrition Bulletin, 31(2 Suppl), S108-116. https://doi.org/10.1177/15648265100312S203