Today’s guest post is written anonymously by a practicing emergency physician board-certified by the American Board of Emergency Medicine. He works primarily in the ER of a women’s hospital in a large metropolitan area and has experience with first- and second-trimester pregnancy emergencies. He routinely uses bedside ultrasound in a broad variety of applications.
Recently there has been a trend in the abortion advocacy community of using medical terminology to draw arbitrary lines regarding when an embryo or fetus’s humanity begins. To some extent the pro-life community also plays into this trend, particularly with laws designed to limit abortion access based on milestones in fetal development (awareness, purposeful movement, nerve development, heartbeat). Every line the pro-life community draws is then countered by pro-choice arguments that those lines don’t truly matter. So as heartbeat bills become a more popular tactic, we see increasing resistance from abortion advocates and the press to the idea that fetal cardiac activity represents something meaningful. Despite using scientific-sounding terms, this resistance often relies on arguments that are scientifically inaccurate or misleading.
A recent New York Times article by Roni Caryn Rabin makes several of these misleading assertions. Among these are that cardiac activity is not a predictor of a successful pregnancy (it is) and that early cardiac motion (characterized here by the familiar “whoosh” sound associated with ultrasounds) is not meaningful and is distinct from a post-birth infant heartbeat. The science of Rabin’s article is more than a little off. To understand this, we need to know more about ultrasound technology, embryology, and fetal development.
Cardiac activity isn’t quite the earliest definitive sign of pregnancy by ultrasound; that would be the gestational sac. However the gestational sac can be difficult to see and define very early in pregnancy. Fetal heartbeat is easier for professionals and laypeople to view and understand. Cardiac activity is first found at 6-7 weeks of gestational age. When referring to fetal cardiac activity detected at 6-7 weeks, Rabin’s article repeatedly refers to “the heartbeat heard on ultrasound” and claims:
The consensus among most medical experts is that the electrical activity picked up on an ultrasound at six weeks is not the sound of a heart beating and does not guarantee a live birth. The sound expectant mothers hear during a scan is created by the machine itself, which translates the waves of electrical activity into something audible.
First trimester ultrasound doesn’t typically involve audible signals.
Here she is likely referring to this sound which is broadly associated with ultrasound in the public consciousness.
However, ultrasound machines do not make this noise in all applications, and especially not commonly in the first trimester, even when detecting cardiac activity. In fact, I chuckled while recently watching Luke Cage when an ultrasound was used to localize a bullet fragment in the abdomen (something we wouldn’t use ultrasound for) and there was the familiar “whoosh whoosh” sound without any heart or blood vessel on screen.
Doppler ultrasound produces the audible signal of a heart pumping blood.
Ultrasounds make the “whoosh” sound only in a certain mode: Doppler. Doppler uses higher energy sound waves to detect and measure motion of tissue or fluid. If the ultrasound machine isn’t using Doppler, it won’t make a “whoosh” sound no matter what it’s measuring. Additionally, Doppler ultrasound is rarely and cautiously used in the first trimester due to the risk of high energy waves harming the embryo or fetus (1999 source, 2019 source). While Doppler does have some first trimester applications, it is rarely used for routine detection of fetal heart rate.
M-mode, not Doppler, is the ultrasound mode typical for first trimester scans.
For first-trimester cardiac activity detection, we use a different, and silent, method called M-mode. M-mode takes one slice of the ultrasound signal and measures changes in the signal over time in order to calculate fetal heart rate, separation of valves throughout the cardiac cycle, and other applications requiring measurement of change over time.
Rabin goes on to claim that the sounds heard during fetal heart rate monitoring are not actual heartbeats, but it’s unclear why she thinks so. The sound from fetal heart rate monitoring is an audible translation of motion: it is the heart chamber walls rhythmically contracting as the heart pumps blood through the embryo’s body. Rabin does not explain why that process would not count as a heartbeat.
Neither Doppler nor M-mode ultrasounds measure electrical activity.
Rabin also implies that the cardiac activity detected by ultrasound in the first trimester is electrical. We have seen above that, when present, Doppler ultrasound detects motion, not electrical activity, and uses measurements of the fetal heart movements to calculate heart rate in the first trimester. While we can measure the electrical activity of the heart from outside the body in humans after birth (the electrocardiogram), this method is nearly impossible during pregnancy due to the mother’s interfering cardiac signals. Fetal cardiac electrical activity can be measured prior to birth, but only by an electrode attached directly to the baby’s head through the cervix to get a more accurate measurement during high-risk deliveries. It is not measured through transabdominal or transvaginal ultrasound.
The fetal heart isn’t a fully developed adult heart; it is a developing fetal heart.
Rabin further describes the embyologic development of the heart. This summary I do not largely disagree with, in technical terms. Yes, the shape and functional output of a fetal heart are different at 6 weeks than at 20. When Rabin states
what the law defines as the sound of a heartbeat is not considered by medical experts to be coming from a developed heart
she is technically correct, but misses another point entirely. Ultrasound findings refute the argument that early detectable cardiac activity is trivial. There must be either a meaningful motion of tissue or fluid to produce a detectable change in M-mode or a higher energy sound wave to produce the “whooshing” sound in Doppler. In order for either ultrasound mode to produce the relevant auditory signals, the fetal heart must be present and pumping blood.
[Read more – Responding to 7 pro-choice claims about embryonic hearts]