### Description

During the first trimester the crown-rump length of the fetus in mm can be used to fairly accurately determine the gestational age.

Menstrual Maturity

Crown-Rump Length in mm

Weeks

Days

Mean

2 SD

6

2

7.0

3.3

6

3

6.5

1.4

6

4

7.0

4.6

6

5

6.5

4.2

6

6

10.0

2.6

7

0

9.3

2.3

7

1

10.3

8.0

7

2

11.8

5.7

7

3

12.8

4.8

7

4

13.4

6.7

7

5

15.4

3.6

7

6

15.4

4.4

8

0

17.0

4.9

8

1

19.5

5.7

8

2

19.4

6.2

8

3

20.4

5.0

8

4

21.3

3.8

8

5

20.9

2.4

8

6

23.2

3.6

9

0

25.8

6.0

9

1

25.4

4.6

9

2

26.7

4.4

9

3

27.0

2.8

9

4

32.5

4.2

9

5

30.0

10.0

9

6

31.3

5.5

10

0

33.0

7.2

10

1

33.8

7.6

10

2

35.2

7.3

10

3

36.0

7.9

10

4

37.3

9.7

10

5

43.4

7.7

10

6

40.1

7.1

11

0

46.7

6.1

11

1

43.6

7.2

11

2

47.5

6.2

11

3

48.8

5.9

11

4

49.0

9.5

11

5

54.0

9.8

11

6

56.2

9.5

12

0

58.3

9.4

12

1

56.8

7.2

12

2

59.4

6.6

12

3

62.6

8.6

12

4

63.5

9.5

12

5

67.7

6.4

12

6

66.5

8.2

13

0

72.5

4.2

13

1

69.7

8.5

13

2

73.0

15.1

13

3

77.0

8.5

13

4

NA

NA

13

5

NA

NA

13

6

76.0

5.7

14

0

79.6

7.8

from Robinson (1975)

An equation based on this data is:

days of gestation =

= ((8.052 * ((crown rump length in mm)^(0.5))) + 23.73) +/- 4.7 days

where

• the variation of +/- 4.7 days could be reduced to +/- 2.7 days if three independent measurements were made

• age range is 6 to 14 weeks

Reece (1987) developed the following equation for first trimester pregnancies:

gestational age =

= 5.23 + (2.53 * (crown rump length)) - (0.345 * ((crown rump length) ^2)) + (0.0172 *  ((crown rump length) ^3))

The units for the crown-rump length are not given, but the equation only works if the crown rump length is given in cm. Figure 4 shows the units in mm, but the results do not correlate with Robinson's tabular data above. The gestational age range covered is 6-12 weeks.

Limitations:

Fetal positioning may make getting an accurate sagittal section difficult. Variation between different instruments may cause variation in readings.