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