Calculation of biometric percentiles at various gestional ages.
What does a fetal ultrasound measure?
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 borrowed from the early 20th century embryologists who found that preserved specimens of early miscarriages assumed a "sitting in the chair" posture so they substituted the head-to-butt length (crown-rump length) as a reproducible method of measuring the fetus. The CRL measurement is 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. An important point to note is that when the due date has been set by an accurately measured CRL, it should not be changed by a subsequent scan. For example, if another scan done 6 or 8 weeks later says that one should have a new due date which is further away, one should not normally change the date but should rather interpret the finding as that the baby is not growing at the expected rate.
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 growth of the fetus. AC measurements should not be used for dating a fetus.
Estimated Fetal Weight (EFW)
The weight of the fetus at any gestation can also be estimated with great accuracy using polynomial equations containing the BPD, FL, and AC. For example, a BPD of 9.0 cm and an AC of 30.0 cm will give a weight estimate of 2.85 kg. To calculate the estimated fetal weight, four measurements (AC, BPD, HC, FL) are entered into a mathematic formula. There are many different formulas for weight estimation, and even the best ones have a 15% margin of error in 85% of all fetuses. In other words, about one in seven fetuses will weigh 15% more or less than estimated.