Hello, Gestational age depends on the period of ovulation. If your wife has irregular cycles, it is difficult to calculate the age of pregnancy by dates and in such an event, there will be disparity between your dates and the sonogram dates. It is true that fetal cardiac activity can be seen at 5-6 weeks of pregnancy through a trans-vaginal sonogram. If it is not visible, repeat sonogram after 10-14 days usually shows it. This can be coupled with serial serum beta-hCG titre assays if the viability of the fetus is in doubt. Doubling titres every 2-3 days suggest good progress. Hope this helps.