Pregnant women who receive an H1N1 vaccine are unlikely to experience higher risks of birth defects, according to a study carried out by researchers in Sweden.

Published in the journal Annals of Internal Medicine, the study involved population-based research on children born from Oct. 1, 2009 to Oct. 1, 2011 to women who were administered Pandemrix, a monovalent AS03-adjuvanted H1N1 vaccine while they were pregnant. Dr. Jonas Ludvigsson and colleagues took into account familial factors to evaluate the kind of risk the children had for birth defects.

Of the 40,983 children who were part of the study, 7,502 were exposed to the vaccine within the first 8 weeks of gestation while 14,385 of the children were exposed during their first trimester. The subjects were then compared to 197,588 unexposed children, many of them siblings.

Based on the researchers' findings, 2,037, or 4.97 percent, of the exposed and 9,443, or 4.78 percent, of the unexposed children had birth defects. The exposed and unexposed children had a corresponding risk difference of 0.10 percent for those with mothers who were vaccinated during their first 8 weeks, and 0.16 percent for those who were exposed in their first trimester.

When they compared siblings, the researchers were not able to find a statistically relevant difference in risk.

While it's definitely good to hear that the H1N1 vaccine is an unlikely cause of birth defects, the researchers acknowledged limitations in their study, such as only data on live births were used. If information on induced abortions and miscarriages were included, it is possible that results may be different. As such, further study is needed on the topic to fully identify the effects of the H1N1 pandemic influenza vaccine on fetal development.

It is important to examine the risks of vaccines administered to pregnant women during the first trimester because this is when early organ development happens, with embryos particularly vulnerable to various agents.

Within the same period though, another study has found that it is fine for pregnant women to get MRIs, if needed. Earlier works have presented the procedure to be safe for both fetus and mother during the second and third trimester but this is the first time that the first trimester has been specifically cleared for MRI use.

In the United States, birth defects are present in one in every 33 babies born, accounting for 20 percent of all infant-related deaths. They can manifest in every part of the body, and 10 percent of cases observed at birth can be traced back to a specific nutritional, biologic, drug and environmental factor that led to the defect. Just 20 percent of birth defects are associated with chromosomal changes or genetics.

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