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Fetal ultrasound measurements can show how the baby is growing and detect abnormalities. During pregnancy, many different ultrasounds measurements can be done. Fetal ultrasound measurements can include the crown-rump length (CRL), biparietal diameter (BPD), femur length (FL), head circumference (HC), occipitofrontal diameter (OFD), abdominal circumference (AC), and humerus length (HL), as well as calculation of the estimated fetal weight (EFW).
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Which ultrasound and sonogram fetal measurements can be calculated during pregnancy?
babyMed has the most extensive list of ultrasound and sonogram tools and calculators available. The ultrasound shows many critical pregnancy structures and embryo measurements—eight essential features are described below.
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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, usually by 6 gestational weeks of the 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 early pregnancy, either inside the uterus or as an ectopic pregnancy outside the uterus. 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.
Try our fetal growth calculator!
Yolk sac
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. Measurements of the yolk sac's size and shape are important when assessing the pregnancy.
Fetal pole
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 crown-rump length measurement is the distance between the top of the embryo and its rump. It can be measured between 7-13 weeks of the pregnancy and gives an accurate estimation of the gestational age. Dating with the CRL can begin 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)
This is the diameter between the two sides of the head and 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. Calculate the fetal measurements here.
Head circumference (HC)
The head circumference or HC measures the circumference of the fetus' head. The HC is usually done after 13 weeks of pregnancy.
Use our head circumference calculator!
Femur length (FL)
This 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.
Femur length calculator
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.
Calculating 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. The babyMed estimated fetal weight calculator will calculate for you the range of weights based on different formulas.
The table below shows typical measurements seen during each week, starting at 4 weeks of gestation.
Ultrasound Measurements in Pregnancy: Measurements (in mm) represent the mean values for each gestational age week.
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Gestational Age (Weeks)
Sac Size
CRL
BPD
Femur Length
Head Circumference
Abdominal Circumference
4 weeks
3
5 weeks
6
6 weeks
14
7 weeks
27
8
8 weeks
29
15
9 weeks
33
21
10 weeks
31
11 weeks
41
12 weeks
51
21
8
70
56
13 weeks
71
25
11
84
69
14 weeks
28
15
98
81
15 weeks
32
18
111
93
16 weeks
35
21
124
105
17 weeks
39
24
137
117
18 weeks
42
27
150
129
19 weeks
46
30
162
141
20 weeks
49
33
175
152
21 weeks
52
36
187
164
22 weeks
55
39
198
175
23 weeks
58
42
210
186
24 weeks
61
44
221
197
25 weeks
64
47
232
208
26 weeks
67
49
242
219
27 weeks
69
52
252
229
28 weeks
72
54
262
240
29 weeks
74
56
271
250
30 weeks
77
59
280
260
31 weeks
79
61
288
270
32 weeks
82
63
296
280
33 weeks
84
65
304
290
34 weeks
86
67
311
299
35 weeks
88
68
318
309
36 weeks
90
70
324
318
37 weeks
92
72
330
327
38 weeks
94
73
335
336
39 weeks
95
75
340
345
40 weeks
97
76
344
354
41 weeks
98
78
348
362
42 weeks
100
79
351
371
References:
Gestational age is measured in weeks from the first day of the last menstrual period and assumes ovulation on day #14 of that cycle.
Gestational sac size is based on the studies of Baltzer FR, et al: Am J Obstet Gynecol,146:973, 1983
BPD measurements are from outer to inner skull tables and are derived from Sabbagha RE and Hughey, Obstet Gynecol 52;402, 1978.
Head circumference measurements are adapted from Hadlock FP, et al: AJR 138:649, 1982
Femur length measurements are adapted from Hadlock FP, et al: AJR 138, 875, 1982
Abdominal circumference measurements are adapted from Hadlock FP, et al, AJR 139:367, 1982
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