What is the fetal biparietal diameter?
The fetal biparietal diameter is the distance between the parietal process, which is actually the transverse diameter of the fetal head.
Biparietal diameter is measured by ultrasound by measuring the distance between the two parietal bulges on a standard plane, generally choosing the thalamus plane, which measures the extension of the skull, generally using the distance between the outer edge of the proximal skull and the inner edge of the distal skull to measure. The scalp tissue is not included, because there are thick and thin scalp tissues that can produce gaps. The moderate size at full term is approximately 9.3 cm.
Note: The measured value is related to the shape of the head, whether the fetus is in the pelvis and fetal position, etc. It is also related to the ultrasound doctor’s experience, so to reduce the error, you can take the average of three measurements.
What is the standard comparison table of fetal biparietal diameter?
The fetal biparietal diameter chart mainly shows the size of the biparietal diameter at different weeks of gestation. In fact, there are several criteria to evaluate fetal growth and development in more detail.
One of them is the standard deviation of the mean value of the biparietal diameter. If the mean value is greater than two standard deviations, it indicates that the diameter is larger, and if it is less than two standard deviations, it indicates that the diameter is smaller.
Another is the evaluation of the biparietal diameter value in how many percentile of the normal value. The overall size of the head is mainly determined by the circumference of the head, not the biparietal diameter, the large head that the fetal head is larger refers to the value of the circumference of the head in the 97th percentile or more, the small head is below the 3rd percentile.
Before 13 weeks, the size of the fetus is still mainly determined by the parietal-rump diameter, and only after 13 weeks does the biparietal diameter start to be measured.
Note: To evaluate the size of the biparietal diameter, we must first check the gestational week, if the ultrasound result is larger than the actual gestational week, we may need to pay attention to whether the biparietal diameter is large, and also pay attention to the head circumference, generally the biparietal diameter measurement from 12 to 28 weeks is closest to the gestational week, which is also a strong guide to the gestational week, in late pregnancy, the biparietal diameter measurement will deviate, and the evaluation of the size of the fetal head is not as accurate as the head circumference indicator.
Is there a relationship between fetal biparietal diameter and mental development?
Some people say that children with larger heads are smarter, but this is actually unscientific. There is no large sample of research studies that show that larger heads are smarter. The most intelligent people have less than ten percent of their brains developed, and the reserve of brain development is sufficient, and there is no necessary connection with whether the head is big or not.
From the point of view of childbirth, it is certainly not desirable to have a large biparietal diameter, and a large biparietal diameter, if caused by hydrocephalus or brain masses, can affect the development of intelligence.
What are the possible causes of a large fetal biparietal diameter?
In a normal state without pathology, a large fetal biparietal diameter may first be related to the shape of the head, which is oval, symmetrical on both sides, and wide at the occipital area. For example, if you have a short head shape, the biparietal diameter will be larger. Second, there is a relationship with race or family. Some people with large heads are not necessarily abnormal, but should be judged in conjunction with the size of the parents’ generation head shape and body shape. Thirdly, if the weight of the pregnant woman is not properly controlled during pregnancy, or if the pregnant woman has gestational diabetes and her blood sugar is not well controlled during pregnancy, the fetus will be large during the whole pregnancy.
If the cause of fetal abnormality is caused by the following cases: it is necessary to consider whether there is hydrocephalus, brain tumor. The width of the ventricles is usually measured at mid-pregnancy to see if the lateral ventricles are dilated.
If the width of the lateral ventricle is close to 10 mm, it is called critical ventricular dilatation; 10 mm-15 mm is mild dilatation, which indicates that the fetus should be further examined to rule out chromosomal abnormalities; if it is greater than 15 mm, there is a possibility of hydrocephalus, which should be monitored dynamically and ultrasound should be done once every four weeks to check if the ventricle continues to dilate. In this case, the peripheral volume remains the same, but there is more fluid in the ventricle, and the brain tissue is more likely to be pressurized, which will have a more serious impact on the child’s intellectual development in the future.
How to determine the fetal development by the fetal biparietal diameter?
Doctors usually use the size of the biparietal diameter to see how the child is growing, to evaluate whether it is in line with the gestational weeks and whether the growth rate is normal. The normal biparietal diameter grows 3 mm per week from 14 weeks to 31 weeks, slows down from 31 to 36 weeks, and grows about 1.5 mm per week, and 1 mm after 36 weeks.
At present, according to the Chinese standard, the biparietal diameter is not used to calculate the weight of the fetus, but according to the information, in foreign countries such as Japan, the biparietal diameter is used as one of the indicators to calculate the weight. The biparietal diameter, abdominal circumference and femoral length are only a diameter line measured under ultrasound, which can only be used as a reference and cannot be completely relied on to estimate the fetal weight. However, we can use the biparietal diameter to estimate the weight. Generally, if the biparietal diameter is greater than or equal to 85mm, it indicates that the fetus is mature, but it also depends on the actual pregnancy week; if it is greater than or equal to 100mm, we should consider the possibility of a huge fetus.
What are the possible adverse effects of a large fetal biparietal diameter?
If the fetus is pathologically large, it may have chromosomal abnormalities, which may affect later intellectual development. The fetal head is the largest diameter in the fetus’ body and is the least malleable, which has a greater impact on delivery.
If the biparietal diameter is large, the labor may be prolonged or the labor may be delayed, resulting in a greater possibility of fetal hypoxia, and vaginal assistance (forceps delivery, etc.) may be required, or perineal tearing, puerperal infection, and postpartum bleeding may occur.
How does a large fetal diploidy affect the mode of delivery?
If the fetal biparietal diameter is found to be large, guidance should be given throughout the pregnancy to prepare for delivery.
After verification of the gestational week.
If a large biparietal diameter is detected in the middle of pregnancy, it is important to monitor the mother’s weight gain, blood sugar and guide the mother’s diet to allow for balanced fetal growth that will help with vaginal delivery.
Because it is the largest, least plastic and most difficult part of the fetus to pass through the pelvis, if the biparietal diameter is indeed large by late pregnancy, a good cephalopelvic score should be done by 36 weeks and after the delivery. Cephalopelvic disproportion can be divided into three cases: normal size fetus with significant pelvic stenosis; large fetus with mild pelvic stenosis; huge fetus with normal pelvic size;
The cephalopelvic disproportion is also related to the position of the fetal head taken, and the trans-pubic sign can be checked from 36 to 38 weeks, if it is positive, it means that the fetal head cannot enter the pelvis, sometimes because the biparietal diameter is large, resulting in insufficient space to enter the pelvis. Some people have a large biparietal diameter but a wide pelvis, so they can have a normal delivery.
What should I do if the fetus has a large biparietal diameter at different weeks of pregnancy?
The treatment of large fetal parietal diameter depends on the factors that cause it, such as maternal, ethnic or familial reasons, and should pay more attention to diet, control the proportion of food eaten and the weekly weight gain. Pregnant women with diabetes should try to abstain from sugar, eat less sweets, and try not to consume carbohydrates other than those necessary for the growth and development of the fetus.
In case of fetal pathology, in addition to diet control, it is necessary to strengthen maternity checkups and to do regular ultrasound to monitor the fetal ventricle and other indicators; if necessary, prenatal diagnosis should be performed to exclude the possibility of fetal congenital anomalies. In late pregnancy, the cephalopelvic score should be done to understand the cephalopelvic relationship and choose a good delivery method.
How to control the fetal biparietal diameter in terms of diet?
If the fetus is found to have a large biparietal diameter and the pregnant woman does not have gestational diabetes, the diet should adhere to the principle of low sugar, low fat and high quality protein intake, and maintain a weight gain of 0.3-0.5 kg per week, with a growth of about 20-25 kg during the whole pregnancy. If the pregnant woman herself has diabetes, it is recommended to see the nutrition department to control the total daily intake of calories, supplemented by appropriate exercise.