Yes, an Ultrasound examination can fail to pick up an ectopic pregnancy if the sac is too small. But it shows up in a few days down the line as the pregnancy and the sac size increase. After one ectopic pregnancy, there is a higher chance of a repeat ectopic and this chance is 4 times higher than a normal woman. So as soon as your wife misses her next period, she should confirm pregnancy by a HCG titre and go for an Ultrasound examination to a good sonologist as soon as possible.