Well this is right up my ally, so to speak, as I have been an ultrasound tech for 15 years, and have done mostly ob/gyn. So I will educate you all a bit. lol
First of all and this is very important, ultrasound is very user dependent, meaning how much and what kind of training the tech has had, or a doctor even for that matter(in ultrasound). So if you have a fairly new or inexperienced tech, you are more likely to have more mistakes or issues missed. I am ARDMS, which means I am a registered tech. That means that I had to go thru extensive, very tough exams to get that registry, much like the doctors have to go thru to get their licenses. The American Registry of Diagnostic Medical Sonographers (ARDMS) are trying to get it to where a tech can't perform an ultrasound without this registry, and although it is getting tougher for techs who are not registered, to get jobs, it is still not always a requirement. So, with that said, always check to see if your tech is registered. Also check to see if they are registered in OB/Gyn, because that is a special exam you have to take to be registered in ob/gyn. I know of many techs who are not registered who perform these exams every day. I believe it should be a requirement in order for them to scan.
I seen a post below where someone had a trisomy 18 baby that was said to be normal. All I can say is that you HAD to have an inexperienced tech to miss that. I am very sorry for you loss.
Ok, now to the sex of the baby. Again, experience plays a big role here, but at 16 weeks it is really tough to determine the sex, except boys you can sometimes tell at that age. I have seen boys as early as 14 weeks, but girls you cannot tell that early. Unfortunately some insurances will only pay for one ultrasound, which I think is crazy, I believe you should have one for each trimester, but who am I? Right? lol. But like another tech said, if all the anatomy of the baby isnt seen, then usually that is a good reason to get another one that insurance will pay for. Then, also, there are times when the baby is just in a position that we just cant tell what it is. I have tried and tried on some babies, had mom turn over, and back, and still to no avail. Guess some of them just dont want to "tell".
But, yes, in the world of ultrasound, there still are no certainties, and we, even with lots of experience dont always pick up everything, we try, but sometimes it just isnt possible. Like the woman who had the baby with the small cleft palate inside the mouth. That would be very difficult to pick up if there was no abnormality in the lips.
I hope this helps all of you understand a little more about ultrasound. Always check for experience and for registered techs to make sure you get the best scan possible for you and your baby. :)