The results of the ultrasound are compared with what is expected to be seen at a given gestational age. Gestational age is calculated from the number of weeks since the last period, however this method usually assumes a 28-day cycle with ovulation occurring on the 14th day. Many women have shorter or longer cycles and ovulate later or earlier. As a result, your baby may be younger or older than expected and this may affect what the ultrasound shows. For example, if a woman has a 35-day menstrual cycle, she is likely to ovulate around the 21st day of her menstrual cycle (ovulation usually occurs two weeks before her period starts). If such a woman became pregnant and did an ultrasound six weeks after the date of her last menstruation, then the ultrasound will show a picture characteristic of 5 obstetric weeks. Despite the fact that this is a normal pregnancy, the gestational sac will be small for 6 OB weeks, it is possible that even the embryo and yolk sac will not be visible. A woman and an ultrasound doctor often do not know when exactly she ovulated, and the data obtained can cause unnecessary concern about a non-developing pregnancy. It can be even more difficult to figure out when ovulation occurred two weeks later. The fetal egg may not be visualized in the uterine cavity and it becomes necessary to carry out a differential diagnosis with an ectopic or biochemical pregnancy.
Also, often a woman does not remember exactly when she last had her period and calls the date “at random”. By specifying the wrong day, even with a typical 28-day cycle, during an ultrasound, you can see something completely different from what was expected.
Remember that any uncertainty about the exact date of ovulation can affect what the ultrasound shows up to 11 weeks of pregnancy.
In this regard, sometimes the doctor cannot accurately determine the result based on only one early pregnancy ultrasound and recommends a follow-up ultrasound after 7-10 days. Waiting for a repeat ultrasound can be emotionally very difficult, but it may be necessary to avoid misdiagnosis, especially if there is no other information to help the doctor interpret the ultrasound results. In most cases, such an assistant is a blood test for hCG, but its data must also be interpreted in dynamics.
The American Pregnancy Association guidelines state that if the average internal diameter of the fetal egg is greater than 16-18 mm and does not contain an embryo complex, or if the CTE of the embryo is greater than 5 mm and it does not have a heartbeat, then such a pregnancy should be regarded as non-developing. In our country, more cautious recommendations have been adopted: the SVD of the fetal egg should be more than 25 mm and the CTE of the embryo should be more than 7 mm.