Hello dear,i understand your concern.In my opinion straining after sex might not effect the conception.Almost most of the women dont know they are going to get pregnant and do their works normally till the pregnancy is confirmed.Once the pregnancy is confirmed one has to be careful like avoid physical strain,avoid lifting heavy weights,avoid intercourse etc.So heavy stress might lead to miscarriage during the initial weeks or months of preganncy but not before conception. Best regards...Dr.Srilatha