PROLONGED PREGNANCY (POSTTERM) (Part One)

Definition

A prolonged pregnancy or postterm is defined as a pregnancy that lasts over 42 weeks or 294 days, starting from the first day of the last cycle in a woman who has a regular menstrual cycle.

When the first day of the last menses is uncertain, irregular, a woman using oral contraceptives, or who has bleeding in the first trimester of pregnancy should take into consideration not only the first day of the cycle but also the regularity and duration of the cycle, as well as an ultrasonographic assessment. Ultrasonographic assessment in the first trimester evaluating CRL varies by 3-5 days of gestational age error, from weeks 12-20 varies by 7-10 days, from weeks 20-30 varies by 2 weeks, and after 30 weeks varies by 3 weeks. Meaning, the earlier an ultrasonographic assessment is done, the smaller the error in estimating the end of a pregnancy.

Incidence

The incidence varies based on population characteristics and local management practices. The incidence varies from 7-12% of all pregnancies. About 4% of all pregnancies go beyond 43 weeks.

Studies show that a BMI>25 for pregnant women and nulliparous women is associated with a significant increase in prolonged pregnancies.

The risk increases by 10-27% when the first birth is postterm and by 39% when there are 2 consecutive postterm births.

The risk increases 2-3 times in a woman whose mother had a postterm pregnancy. Maternal genetic and non-paternal factors influence postterm pregnancy.

Risk factors

Factors that increase the risk for a prolonged pregnancy are:

• Obesity (a modifiable factor with diet, physical activity, and medication)
• Primiparity
• Previous postterm pregnancy
• Male sex of the fetus
• Genetic factors (maternal genetic factors influence about 30% and not the paternal ones)
• Hormonal factors (deficiency of placental sulfatase, a rare recessive disorder linked to the X chromosome)

Pathogenesis

The pathogenesis of postterm pregnancy is not fully understood. Birth involves 3 mechanisms of interaction: hormonal, mechanical, and inflammatory processes (the placenta, mother, and fetus play a vital role). Placental production of CRH (corticotropin-releasing hormone) is directly linked to the duration of pregnancy. The synthesis of CRH increases exponentially over the course of pregnancy and peaks at the time of birth.

In postterm pregnancy, there are changes in the biological mechanisms that regulate the extension of pregnancy. This is linked to a genetic predisposition in the CRH genes. It is possible that the maternal phenotype also changes the maternal tissue response to usual hormonal stimuli, such as in obesity.

CRH affects the direct fetal stimulation in the production of adrenal DHEAs, which serves as a precursor for the synthesis of placental estradiol. The synthesis of estradiol decreases and the synthesis of progesterone, which inhibits uterine contractions, increases leading to an extension of pregnancy.

Causes

• Incorrect assessment of the menstrual cycle, most common
• Maternal obesity
• Previous postterm pregnancy
• Anomaly in maternal central nervous system
• Placental insufficiency for the enzyme sulfatase
• Anencephaly (fetal skull malformation)