@Franklin58200 Right, but for neither pregnancy did my wife do a transvaginal. She just did normal ultrasound. Why would a transvaginal be needed?
Ultrasound is always a trade off. Higher frequency transducer equals much higher resolution thus clearer pictures and information of smaller structures however higher the frequency the less distance through tissue that ultrasound beam will travel.
Lower frequncy ultrasound will travel further at expense of resolution.
Being in the vagina or rectum is some cases the beam is much closer to ovary, endometrium, gestational sac etc. Transvaginal ultrasound modern scanner typically around 9mhz to abdominal probe normally 4-6mhz.
At your 12 week scan everything is easy to see on abdominal scan but at 4-5 sometimes externally see nothing or maybe an empty appearing gestational sac but transvaginal scan can see yok sac, fetal pole and even cardiac pulsations.
Can help confirm early date, location of pregnancy and number.
Worked in family practice so no radiologist but this simplist understanding, someone here can prob provide better explanation.
If @FlauscheBall had a small polyp causing imb it can be seen via internal ultrasound and missed on abdominal scanning.