Mailbag 35: What Does the Fox Say? with Dr. Nathan Fox

In this episode of the Healthful Woman podcast, Dr. Nathan Fox opens up the mailbag again, answering listener questions about some of the topics that come up most often in high-risk pregnancy care: how doctors decide when to repeat Doppler testing on twins, how twin growth is measured differently than singleton growth, and how patients can watch for preeclampsia between prenatal visits. Each question comes from a real listener, and Dr. Fox walks through the reasoning doctors use behind the scenes.

How Often Should High-Risk Twins Be Monitored?

The first question comes from a listener named Beth, whose twins showed elevated blood flow resistance and absent diastolic flow on umbilical artery Doppler testing. She wanted to know how doctors decide the safe interval between Doppler tests once a result comes back abnormal.

Dr. Fox explains that umbilical artery Doppler measures how much resistance blood meets as it flows through the umbilical cord, and that abnormal results generally progress through three stages: an elevated systolic-to-diastolic (S/D) ratio, absent end-diastolic flow, and, most concerning, reversed flow. There’s no single fixed schedule that applies to every patient. As Dr. Fox puts it, “We decide this literally person by person based on the situation,” weighing how abnormal the Doppler is, the gestational age, and how the babies are doing on other measures like non-stress tests. In Beth’s case, that meant twice-weekly Dopplers and three non-stress tests a week, with her twins ultimately delivering at 34 weeks after a period of close surveillance.

Understanding Twin Growth and Cervical Length Screening

The second question, from a listener named Matt, has two parts: how twin growth compares to singleton growth in the third trimester, and how cervical length measurements work alongside fetal fibronectin (FFN) testing to assess preterm birth risk.

Dr. Fox notes that twins typically measure smaller than singletons late in pregnancy, and that there’s an ongoing debate in the field over whether this reflects a true growth restriction or simply the normal, adaptive physiology of a twin pregnancy. Because of that uncertainty, most U.S. practitioners still use standard singleton growth curves when evaluating twins, rather than twin-specific curves. On the second part of the question, cervical length and FFN testing are both used to help stratify a patient’s risk of preterm birth, giving both patient and physician more information to plan around. Dr. Fox’s philosophy on screening in general: “Better to know than not know,” comparing it to checking the weather before deciding whether to bring an umbrella. For twin pregnancies specifically, the practice also offers a risk calculator at mfmnyc.com/twin for patients who want to look at their own numbers.

Catching Preeclampsia Between Visits

The final question comes from a new patient who asked how preeclampsia is detected between prenatal appointments, and whether she should be monitoring her blood pressure at home.

Dr. Fox points out that this is exactly why visit frequency increases as pregnancy progresses — blood pressure monitoring is one of the main reasons behind that schedule. The tricky part, he explains, is that “most people, when their blood pressure starts going up, they will not know it’s happening,” since rising blood pressure typically doesn’t come with obvious symptoms. Home blood pressure monitoring can help close that gap between visits, but it isn’t something every patient needs. It tends to be recommended more for those at higher risk, such as patients with a prior history of preeclampsia or existing hypertension on medication. Since protocols vary by practice, Dr. Fox encourages any patient concerned about preeclampsia to bring it up directly with their care team.

Learn More

Listen to the complete episode, “Mailbag 35: What Does the Fox Say?” on the Healthful Woman podcast to hear more of Dr. Fox’s answers to these listener questions, including more of the reasoning behind each recommendation. For high-risk pregnancy care in New York, contact Maternal Fetal Medicine Associates today by completing the online contact form.

Maternal Fetal Medicine blogs are intended for educational purposes only and do not replace certified professional care. Medical conditions vary and change frequently. Please ask your doctor any questions you may have regarding your condition to receive a proper diagnosis or risk analysis. Thank you!