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Fetal and Obstetric Ultrasound Measurements in Pregnancy

Obstetric Ultrasound Types and Information

Fetal body obstetric ultrasound measurements reflect whether the fetus has grown enough or whether the measurements fall outside the normal range. The following measurements can usually be made:

Gestational sac (GA)

The gestational sac (or gestation sac or GA) is an intrauterine structure surrounding the pregnancy that 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.

Crown-rump length (CRL)

The CRL measurement is the distance between the top of the embryo and it's 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. (Chart and further comments) Dating using the BPD should be done as early as is feasible.

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. (Chart and further comments) 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. computer softwares and lookup charts are readily available. For example, a BPD of 9.0 cm and an AC of 30.0 cm will give a weight estimate of 2.85 kg.

Fetal and Obstetric Ultrasound Measurements in Pregnancy: Measurements (in mm) represent the mean values for each gestational age week.

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

Obstetric Ultrasound specifics:

  • Gestational Age is 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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