This paper seemed really cool at first glance but then I was disappointed when I read it. It basiacally supports the hypothesis that during the first trimester of pregnancy in-group favoritism is at the most heightened state of the whole pregnancy because this is when the mother's immune system is on its highest alert.
The major thing that I did not like about it was that the title says ethnocentrism but then the research looks at American vs. non-American, so not so much at ethnicity, per se, after all.
Elevated ethnocentrism in the first trimester of pregnancy
Carlos David Navarrete, Daniel M.T. Fessler and Serena J. Eng
Evolution and Human Behavior Volume 28, Issue 1 , January 2007, Pages 60-65
Abstract: Recent research employing a disease-threat model of the psychology of intergroup attitudes has provided preliminary support for a link between subjectively disease-salient emotional states and ethnocentric attitudes. Because the first trimester of pregnancy is a period of particular vulnerability to infection, pregnant women offer an opportunity to further test this association. We explored the expression of intergroup attitudes in a sample of pregnant women from the United States. Consistent with the predictions of the disease-threat model, results from our cross-sectional study indicate that favoritism toward the ingroup peaks during the first trimester of pregnancy and decreases during the second and third trimesters. We discuss this finding in light of the possible contributions of cultural and biological factors affecting ethnocentrism.