Ultrasounds

FAQ

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Question:
My wife has already been for five ultrasounds during this pregnancy as our doctor believes the baby is not growing properly. I’m concerned that this could be harmful for the baby. Do you know where I can find any information regarding the safety of ultrasound during pregnancy? I would also like to know why she needs to be scanned so many times ?

sonography

Answer:
Please take a look at the section on Safety references. There is currently no evidence of a cumulative effect of ultrasound on the fetus. Your doctor is probably trying to monitor the growth of the fetus by taking body measurements at set intervals. If the growth lag becomes serious, he will need to perform some other tests in order to monitor the well being of the fetus and act quickly if the baby needs delivering at an earlier date.

Question:
I heard somewhere that the heart rate of a female fetus is faster than that of a male fetus, with the male fetus heart beating at less than 140 beats per minute. I wondered whether this is true and, if so, when is the best time to count the heart beat to determine the difference?

Answer:
This is one of the great pregnancy myths. There is no ultrasounds or scientific evidence of any difference in heart rate between fetuses of the two sexes.

More Ultrasounds Questions

Question:
I’m 20 weeks pregnant and had an ultrasound scan yesterday. I had one previously at 17 weeks where the ultrasound technologist said she couldn’t see any boy parts, and advised me to come back for a repeat scan. Yesterday she said there definitely weren’t any boy parts down there, and she began to look for the labia. She searched for around ten minutes but couldn’t see the labia.

She said that by 20 weeks they can usually see the penis or scrotum, so she believes that this baby must be a girl. With our two sons, the technicians were able to tell that they were boys at 18 and 20 weeks so I am wondering why it's so difficult this time round. Is it strange to not be able to see the labia at twenty weeks & does that mean that it could still be a male?

Answer:
You are obviously fairly anxious to learn the sex of your baby. In order to be confident about the sex, the sonographer should reassure himself that he can actually see the penis and scrotum in the case of a boy and the labia in the case of a girl.

The absence of the penis cannot be used as sufficient evidence that the fetus is a girl. Your sonographer was being cautious not to make a mistake which could prove extremely embarrassing. In the majority of cases, it should be possible to determine the sex of the baby by about 20 weeks and very often even at 16.

However, the correct visualization of any fetal part depends on a great number of factors such as fetal position, amount of liquor and thickness of the abdominal wall. I myself have had occasions when I could not be certain about the sex of the baby even at 28 weeks.

I have been asked a great number of times about the accuracy of fetal gender diagnosis by ultrasounds. I have to say that it should be quite on point (between 95 and 100%), specifically after twenty weeks, or even sixteen. But I must reiterate, one must remember the sonologist may not always get a good look at the perineum (private parts) of the fetus for a number of reasons and therefore may not be able to provide you with an "accurate" answer.

It is really necessary to ask this question to the technician performing the scan. He or she is the only person who can possibly tell you just how accurate your particular diagnosis is.

Question:
I am 18 weeks pregnant and went for a routine 4d ultrasound yesterday. My doctor told me that my baby has a small cyst in the brain. I have been asked to go for a further scan in two weeks time to find out whether the cyst has grown any bigger, and to determine whether my baby has any other problems. The doctor mentioned chromosomes and I’m worried that my baby is going to be very sick.

Answer:
What your doctor probably saw was a "choroid plexus cyst" in the fetal head. This occurs in about 0.5 to 1 percent of fetuses but the vast majority will eventually disappear. However they have been found to be linked with chromosomal abnormalities, notably Trisomy 18 and trisomy 21 (some 5 to 30 percent of these fetuses have the cysts).

It is probably worse if there is more than one cyst and they are larger than 10mm. Most of the time a single cyst is found to be benign. It is vital that your doctor carefully checks for other features of chromosomal trisomies in the next scan. She may order an amniocentesis if there are any doubts about the findings.

Question:
I would like to know how accurate ultrasounds are as I heard that abnormalities can sometimes be missed.

Answer:
It is impossible to estimate exactly how accurate sonograms are at detecting disabilities, and there can be both false negatives and false positives.
With regard to detecting fetal abnormalities, several factors must be considered:

a) It is not possible to diagnose every problem with ultrasound. Conditions that don’t manifest as a structural or gross functional abnormality (such as a very abnormal heart beat or mental retardation) may not be seen.

b) Not all problems which can be detected will be diagnosed. For instance, in many ultrasound examinations the baby's fingers and toes are not counted, even though it is possible to do so if required.

c) The ability to detect abnormality in the fetus at an ultrasound exam is dependent on a number of factors, including the size and position of the fetus, the amount of amniotic fluid around the fetus, the body habitus of the mother (whether she is fat or slim), the type of equipment used, and, most importantly, the skill and experience of the operator.

d) Some problems (such as anencephaly) are easier to diagnose than others (such as cleft palate).

The accuracy of ultrasounds for dating a pregnancy is dependent on when the ultrasound is taken. Dating a pregnancy is done most accurately during the first eighteen weeks. Taking measurements of the sac at five to seven weeks does not provide accurate results. Measuring the crown-rump length provides an accuracy of plus or minus three days when performed at seven weeks; this test can be used from the seventh to the fourteenth week.

Between 14 and 25 weeks, measures of the biparietal dia., femur length and abdominal circum. are typically used. The accuracy of this method is plus or minus seven to ten days. With advancing gestation age, the level of an ultrasounds accuracy decreases.

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