The method of ultrasound diagnostics is considered one of the most accurate for determining pregnancy in the early stages, but you should not assume that its accuracy is 100%. In gynecology, the accuracy of this study is estimated at about 90%. In early pregnancy, the accuracy decreases to 75%. A doctor is first and foremost a person, not a machine with a program embedded in it. He has the right to make a mistake, especially if a woman has problems with the health of the reproductive system. So, the doctor can confuse uterine fibroids with pregnancy in the initial stages, if the woman had not previously been diagnosed with fibroids, and she found out about her presence only on ultrasound. A cyst or polyp can be confused with a fetal egg, since a cyst is also an echogenic formation.
If a woman had late ovulation, then pregnancy a week after the delay may not be detected at all by an ultrasound specialist, since the fetal egg later descended into the uterus and is not yet visualized. Naturally, the doctor will write in the conclusion that no signs of pregnancy were found, but after 7-10 days on a second study, he will be able to determine both the fetal egg and its structure. Only the dimensions will help to understand that ovulation was really late.