By Taylor Morrison CNM~

Your “due date” came and went, and there are still no signs of labor!

Not to worry. The average length of pregnancy is actually 40 5/7wks for first-time mothers and 40 3/7wks for subsequent pregnancies. Remember that your “due date” is really a “guess date” that provides us a 5-week window in which your baby can be born at full term (37 0/7wks to 41 6/7wks).

However, your healthcare provider may now be recommending some additional fetal testing at the 41wk mark.

Why would we need more thorough monitoring at this point?

We know that some risks increase after 41wks of pregnancy. These include maternal infection, postpartum hemorrhage, Cesarean delivery, low amniotic fluid, meconium-stained amniotic fluid, and stillbirth. Based on these increased risks, some practitioners recommend induction at 41wks. However, the evidence shows that the increases in absolute risk are small enough that it is reasonable to delay induction until 42wks, as long as baby is still receiving adequate oxygen and blood flow from the placenta. For this reason, biweekly antenatal fetal testing is the standard of care for pregnancies between 41 and 42 weeks.

How does the testing work?

If a baby is not receiving adequate blood flow and oxygen from the placenta, he/she will start to conserve oxygen by decreasing energy expenditure and prioritizing blood flow to the more critical organs (brain and heart). A baby who is experiencing oxygen-deprivation will show decreased heart reactivity, decreased movements, and decreased amniotic fluid. We can detect these changes through the non-stress test (via electronic fetal monitoring) and through ultrasound.

Non-Stress Test (NST) Fetal Testing

To perform an NST, your care provider will attach two electronic fetal monitors to mom’s belly. These measure baby’s heartbeat and uterine contractions, respectively. This will provide a print-out of baby’s heart rate similar to an EKG. You provider will be looking for multiple markers on the fetal heart tracing that indicate a healthy, well-oxygenated fetus. In order to “pass” the NST, your baby’s baseline heartrate should be between 110 and 160 beats per minute, the variability should be moderate, two accelerations in the heartrate should occur in a 20-minute window, and no decelerations should be seen. If all of these criteria are met, your baby has passed the NST!

Fetal Testing

Now for the second component…

Amniotic Fluid VolumeFetal Testing

Your care provider will use ultrasound to measure the amount of amniotic fluid surrounding baby. A maximum vertical pocket will be measured and should be 2cm or greater. An amniotic fluid index can be calculated by measuring a pocket of fluid in each of the 4 quadrants of mom’s belly; this should equal 5cm or greater.

If your amniotic fluid is low, there is a concern that chronic oxygen deprivation is causing baby’s body to decrease blood flow to the kidneys, resulting in decreased urine output and low amniotic fluid. Additionally, babies with low fluid are at increased risk for cord compression (due to the reduced “cushioning” of the fluid) leading to an inability to tolerate labor contractions and an increased risk of Cesarean section.

If your amniotic fluid is adequate, this is a good indication that baby is receiving adequate oxygenation and that baby will tolerate labor well.

Fetal Testing

When and where is this testing done?

At Del Mar Birth Center, the NST and ultrasound are performed by the midwife in the clinic! Other care providers may be able to perform this testing in the office or may refer you to a perinatologist or ultrasonagrapher. The testing should be done at 41 0/7wks (exactly one week after your “due date”) and then again 3-4 days later. If you are still pregnant at 42wks, an induction of labor will be recommended due to the increasing risks to mother and baby. Ask your care provider about natural methods of encouraging labor and medical induction techniques.