Question

A study was undertaken to determine the strength of association between several risk factors and the duration of pregnancies. The risk factors considered were mother's age, nutritional status, history of tobacco use, and history of alcohol use. The response of interest, pregnancy duration is a three-categories variable that was coded as follows:

Yi	Pregnancy Duration Category
1	Preterm
2	Intermediate term
3	Full term

We would like to study the association between the duration of pregnancy and risk factors.

(a) Propose an appropriate model and explain how you assess whether your model is adequate

(b) Describe how to estimate the parameters in your model in a step by step manner

(c) We would like to test which risk factors are associated to the pregnancy duration. Explain what test statistic is used and why this test statistic is appropriate. Provide testing procedure in a step by step manner.

(d) We also would like to calculate the prediction accuracy of your model in (a). How would you measure the prediction accuracy of your model? Explain your procedure in a step by step manner.

Answer 1

Answer:

(a) Let's suppose a 25yr old pregnant female with 37 weeks of gestation.

We can assess adequacy of her pregnancy in the following ways:

1) assess hemoglobin levels, hematocrit, blood pressure and cardiac output monitoring.

2)General and abdominal examination.

3)Ultrasonography for localisation of placenta and estimation of amniotic fluid.

4)proper history of any - abnormal vaginal bleeding, or placental abnormalities.

(b)The parameters of this gestation can be assessed by:

1) fetal heart sounds : These can be measured by : auscultation, Doppler ultrasound or cardiotocography(electronic fetal monitoring). It detects hypoxia or any fetal distress and help in planning further management.

2)fundal height: this is measured in order to know the gestational period.

When these parameters are measured , the following possibilities occur:

1: mother hemodynamically stable , no fetal distress : wait for spontaneous labor initiation at 37wks and consider delivery with watchful expectancy.

2: if mother is - hemodynamically not stable or bleeding actively or fetal distress is present then, induction of labor is preferred, applying resuscitative measures and oxytocin infusion.

(c) Risk factors associated with pregnancy duration are:

For pre term pregnancy : -infections , history of smoking , abnormal fetal presentation, oligohydramnios.

For post term pregnancy: -maternal obesity ,previous history of post term pregnancy or CNS abnormalities.

The test statistic used to measure gestational age is : Fundal height

It can be done as follows:

The distance between the umbilicus and the ensiform cartilage is divided into three parts, The fundal height corresponds to the junction of upper and middle thirds at 32wks, up to the ensiform cartilage at 36wks, it comes down to 32week level at the 40th wks because of engagement of presentating part. Hence engagement is tested, if the head is floating ,then it's 32wks pregnancy and if engaged then it's 40wks pregnancy.

This is appropriate than other methods because they have some limitations.

(d) To measure prediction accuracy, different methods can be compared for their results: for example : Fundal height measurement is more accurate than sonography, gestational age is measured by biparietal diameter, head circumference, abdominal circumference and femur length which is less accurate and assessment can be done only if accurate dating scan has been done.