What is femur length (FL)?
Femur length measures the longest bone in the body and reflects the longitudinal growth of the fetus. Its usefulness is similar to the biparietal diameter (BPD, which is the diameter between the 2 sides of the head.
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Femur length increases from about 1.5 cm at 14 weeks to about 7.8 cm at term. (see chart) Similar to the BPD, dating using the femur length should be done as early as possible.
Why are fetal measurements important?
First of all, an ultrasound can help in determining a due date. Based upon the results of the measurements, the gestational age of the baby will be calculated based upon the average size of other babies scanned during the 20th week of pregnancy. However, the range of calculations at this time of the pregnancy is within 2-3 weeks of the actual due date, while an earlier ultrasound is more reliable to calculate the due date.
However, more importantly, if any fetal abnormalities are found during an initial ultrasound, further investigation can be performed and if any treatments are warranted, they can begin early on.
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What are the most important fetal measurements?
Gestational sac (GS)
The gestational sac (GS) is a structure that surrounds an embryo and is likely the first pregnancy structure that can be seen on ultrasound in early pregnancy. The GS encloses not only the embryo but also the amniotic fluid, which helps to nourish and protect the developing baby. The gestational sac is the structure ultrasound technicians look for when they need to confirm the presence and viability of an early pregnancy. It can be used to determine if an intrauterine pregnancy (IUP) exists prior to the visualization of the embryo. It can be measured across and the weeks of the pregnancy can be determined with about 5-day accuracy.
The yolk sac is a membranous sac attached to the embryo. It can be seen on ultrasound between the embryo and the gestational sac. The yolk sac functions as a means for the nourishment of the embryo before the circulatory system and the placenta develop.
The fetal pole is amass of cells before the embryo is visible. It grows at a rate of about 1 mm a day, starting at the 6th week of gestational age. Thus, a simple way to "date" an early pregnancy is to add the length of the fetal pole (in mm) to 6 weeks. Using this method, a fetal pole measuring 5 mm would have a gestational age of 6 weeks and 5 days.
Crown-rump length (CRL)
The term crown "crown-rump length" is the measurement of the distance between the top of the embryo and its rump. The CRL can be measured between 7 to 13 weeks and gives an accurate estimation of the gestational age. Dating with the CRL can be within 3-4 days of the last menstrual period.
Biparietal diameter (BPD)
The diameter between the 2 sides of the head. This is measured after 13 weeks. It increases from about 2.4 cm at 13 weeks to about 9.5 cm at term. Different babies of the same weight can have different head size, therefore, dating in the later part of pregnancy is generally considered unreliable.
Head circumference (HC)
The head circumference or HC measures the circumference of the fetus' head. The HC is usually done after 13 weeks of the pregnancy.
Femur length (FL)
Measures the longest bone in the body and reflects the longitudinal growth of the fetus. Its usefulness is similar to the BPD. It increases from about 1.5 cm at 14 weeks to about 7.8 cm at term. (Chart and further comments) Similar to the BPD, dating using the FL should be done as early as is feasible.
Abdominal circumference (AC)
The single most important measurement to make in late pregnancy. It reflects more of fetal size and weight rather than age. Serial measurements are useful in monitoring the growth of the fetus. AC measurements should not be used for dating a fetus.