### Description

The risk for meningomyelocele and anencephaly during pregnancy can be estimated from the maternal serum AFP if the population and gestational age are known.

Starting data:

(1) maternal serum AFP as multiple of mean (MoM) adjusted for weight and other factors

(2) gestational age in weeks, from last normal menstrual period, valid for weeks 15 to 20

(3) data for risks of various conditions for location and population

Conditions for which risks calculated:

(1) unaffected pregnancy

(2) twin pregnancy

(3) open spina bifida

(4) anencephaly

Step 1: Calculate Patient's a priori Risks

The risk for conditions should be as specific as possible. If local data is not available, general data from the country can be used. The risk is expressed as a decimal.

Condition

Default Base Risk in USA

Risk as Decimal

twin pregnancy

1 in 80

0.0125

open spina bifida

1 in 1800

0.0005556

anencephaly

1 in 1800

0.0005556

For open spina bifida, the presence of certain conditions can increase the relative risk for neural tube defect during a pregnancy. If one or more of these conditions is present, then:

risk for open spina bifida =

= (base risk) * (factors present in table below)

Conditions affecting the a priori relative risk for neural tube defect

Factor

first degree relative (parent, sibling)with open neural tube defect

26.7

second degree (aunt, uncle, niece, nephew) relative with open neural tube defect

3.7

third degree relative (first cousin) with open neural tube defect

1.4

parent with scoliosis

5

sibling with hydrocephalus

20

sibling with tracheoesophageal fistula or cleft lip or cleft palate

3

sibling with exstrophy of urinary bladder

6

sibling with diaphragmatic hernia

8

sibling with renal agenesis

4

maternal insulin-requiring diabetes mellitus

4

maternal valproate therapy

20

risk unaffected =

= 1.00 - ((risk for twin pregnancy) + (risk for open spina bifida) + (risk for anencephaly)

Step 2: Calculate the Standard Log-Gaussian Ordinates

standard log-gaussian ordinate for each condition =

= (LOG10 (AFP MoM) - (mean for condition)) / (standard deviation for condition)

Note: Data for twins not listed in Table 39-6 page 2125 (Ashwood, 1994).

mean for conditions

week gestation

unaffected

twins

open spina bifida

anencephaly

15

0.0000

0.4581

0.6262

16

0.0000

0.5579

0.7510

17

0.0000

0.6063

0.8274

18

0.0000

0.6034

0.8554

19

0.0000

0.5491

0.8349

20

0.0000

0.4434

0.7660

standard deviations for conditions

week gestation

unaffected

twins

open spina bifida

anencephaly

15

0.1976

0.2697

0.3342

16

0.1976

0.2697

0.3342

17

0.1976

0.2697

0.3342

18

0.1976

0.2697

0.3342

19

0.1976

0.2614

0.2585

20

0.1976

0.2614

0.2585

Step 3: Calculate the Relative Frequencies for Each Condition

relative frequency for each condition =

= EXP ((-1) * ((standard log-gaussian ordinate for condition) ^2 ) / 2) / (2 * π * (standard deviation for condition)

standard deviations for conditions

week gestation

unaffected

twins

open spina bifida

anencephaly

15

0.1976

0.2697

0.3342

16

0.1976

0.2697

0.3342

17

0.1976

0.2697

0.3342

18

0.1976

0.2697

0.3342

19

0.1976

0.2614

0.2585

20

0.1976

0.2614

0.2585

Step 4: Determine Patient's posterior Risks

risk for condition =

= (a priori risk for condition) * (relative frequency for condition) / SUM ((a priori risk for condition) * (relative frequency for condition))

The risk for twins is excluded if mother is known to be carrying twins.