The embryonic heart rate (EHR) has been found to correlate with gestational age by crown-rump length (CRL) in normal pregnancies. This research compares the EHR and CRL for a population (n=173) in an assisted fertility program, and a population (n=173) of normal pregnancies which were paired by CRLs.
Since the advent of sonography, and specifically endovaginal sonography, a window on the developing human embryo and fetus has been opened that was not available before. It is known that the normal embryonic heart rate (EHR) accelerates until the 9th menstrual week (LMP). [1, 2 ,3 , 4, 5] The heart rate in the early pregnancy has a characteristic acceleration phase, peak rate of 175 B/M (+/-20, 2SD) at approximately 9.2 weeks, followed by a deceleration until approximately 15-17 weeks. After the deceleration phase the heart rate becomes relatively constant after 16 weeks at about 145 B/M (+/-25), with a very slight negative slope, until term.
The embryonic heart rate normally exhibits a low short-term, beat-to-beat, variability [4, 6]; while the rates in the last half of pregnancy have greater beat-to-beat variability. Even though the embryonic has low beat-to-beat variability in the short term, it has a rapid acceleration and peak.
Pregnancies resulting from ovulation induction or assisted reproductive technologies were monitored by sonography during the embryonic period (< 9.3 weeks). This research was approved by institutional review. M-mode EHRs were measured in assisted and non-assisted gestations. The assisted population is 173 embryos including singleton and multiple gestations (singletons = 73.2%. twins=22.3%, and triplets = 4.5%). The unassisted population is from 9000+ pregnancies that were referred for sonographic evaluation. This population has been described elsewhere . The unassisted population of 173 was selected by randomized selection from the 9000+ gestations by CRLs matched with the assisted population.
Ages were estimated for both populations using the following regression.
CRL age (weeks) = 4.1+(0.00020798*(CRL^2))+(0.94113212*SQRT(CRL))
Analysis of the crown-rump length (CRL) ages before 9.2 LMP weeks and the EHR produced the following regression:AGE in DAYS = 11.5+EHR(0.27)
(p < 0.001, r2 = 0.78).
A modification of this regression formula yielded the following simple relationship :EMBRYONIC AGE in DAYS = EHR(0.3)+6
+/- 2SD (95 %) = +/- 7 days; valid before 9.2 LMP weeks (64 LMP days)
The resulting ages (by CRL & EHR) for both populations were compared by t-tests of paired two sample means and analysis of variances (ANOVA). The following is a plot of the four sets of estimated ages.
There was no statistically significant difference between the EHRs, paired by CRL, for assisted (m = 134.6 B/M) and non-assisted (m = 135.5 B/M) pregnancies (p = 0.41, r = 0.80). There was also no statistically significant difference between the ages estimated using the above regressions for EHR (p = 0.43, r = 0.79,) or CRL (p = 0.55, r = 0.99992), t-test of paired two sample means. By ANOVA there was a small (2 day) but statistically significant difference in the mean estimated ages for the four: EHR Assisted = 6.63 weeks, EHR Non-Assisted = 6.66 weeks, CRL Assisted and Non-Assisted = 6.88 weeks.
This is the first comparison of embryonic heart rates in two different populations. The fact that they both exhibit similar acceleration during the embryonic period is suggestive that this phenomenon is related to the physical development of the heart. Our own follow-up on the 1993 population found that 4 or 5 (1 lost to follow up) of 6 EHR ages that had fallen more than 7 days (- 2SD) behind the CRL ages ended in 1st trimester demise even when the measured heart rate had been > 95 beats/minute. This is a 67% or 83% positive predictive value for spontaneous first trimester abortion. (See graph) Other research has shown that slow heart rates during the embryonic period do have a greater risk of having anomalies.[9,10]
From over 15 years of study of the embryonic heart rate this researcher (TJD) has found the following:
- For most accurate M-mode heart rates, enlarge the image and M-mode tracing as much as possible and use the fastest sweep speed.
- The heart rate accelerates at 3.3 B/M during the first month of beating from around the start of the 5th LMP week until 9.2 weeks.
- The above acceleration is approximately 10 B/M every 3 days.
- The acceleration rate is approximately 100 B/M during the first month of beating.
- If the embryonic age calculated from the EHR is >7 days less than the age by CRL, there appears to be an increased likelihood of 1st trimester failure or anomalies. This difference in age is calculated as: CRL age - EHR age = 7 or more days.
- Doppler is not necessary to establish EHRs, M-mode works well with less acoustic energy.
The sonogram here is a twin gestation at approximately 6 weeks. Both EHRs are shown in the M-mode tracing. The larger embryo (upper) has the faster EHR. These embryo's were followed throughout pregnancy, and maintained appropriate CRL-EHR relationships throughout the embryonic period. After the twins passed the heart rate peak at 9.2 the larger embryo had the slower heart rate. Both were normal at birth.
At the initial examination the mother's reported LMP was 5.9 weeks, and normal. The following table gives the initial embryonic parameters.
|Twins||CRL||CRL age||EHR B/M||EHR age|
|Large||10 mm||7.1 wks||134||6.6 wks|
|Small||6 mm||6.4 wks||117||5.9 wks|
The twins delivered spontaneously in the 34th week. The larger twin was 2700 g, and the smaller 1955 g. Details of the eight sonographic examinations of this twin gestation are published in DuBose TJ; FETAL SONOGRAPHY, Chapter 7 Size/Age Analysis, W. B. Saunders 1996, p. 151-154.
For a complete discussion and mathematical description of the curve of the normal human heart rate throughout gestation see Chapter 12 Heart Rate in DuBose TJ, FETAL SONOGRAPHY. In addition, a remarkable study of sonoembryology including the heart but focusing on the three dimensional development of the fetal brain is given in the doctoral dissertation of Harm-Gerd Karl Blaas. Dr. Blaas's well illustrated and clear discussion is remarkable... highly recommended for any student of contemporary embryology.