Ugghhhhh. I really wish I could just hear the ultrasound tech say, “there’s the heartbeat!” and be super excited and go home and start decorating the nursery and picking baby names.
But that’s not who I am. I’ve been through too much. I’ve seen too much.
Today I was 6w3d. And I got what should be, by all accounts, great news. There’s a heartbeat. My doc’s office does not give a heart rate measurement, but I pressed repeatedly (“is it slow? It looks slow”) to be told it looks fine and not slow. (He said it would be obvious the heart rate was slow if it was below 100bpm. I’m not so sure about that.) The subchorionic hematoma is invisible. (Cured!) The crown to rump length is 5.6mm—right where it should be. (Recall yesterday I said the average was 5.4mm with a range of 3.4mm (5th percent) to 7.9mm (95th percent).) The gestational sac diameter was 20.9mm, right where it should be. (Ave 17.4mm, range 11.7mm (5%)-24.3mm (95%).) And the yolk sac diameter was 2.5mm, far far far below what it should be. (Ave 3.6mm, range 2.7mm (5%) to 4.5mm (95%).)
Wait, what?
You heard me—now I’m worried about my tiny yolk sac.
It literally never occurred to me to worry about the size of my yolk sac until I looked at the “average” and “normal” (ie. 5th-95th percentile) numbers. My yolk sac is TEENY. Well below the 5th percentile.
Cut to me reading whatever I can about small yolk sacs. People tend to agree that below 3mm at this gestational age is an abnormally small yolk sac, but the only articles I could find suggesting increased adverse outcomes were for yolk sacs smaller than 2mm. Interestingly, large yolk sacs (above 6mm) were associated with adverse outcomes.
What the heck?! Well, apparently a large yolk sac can indicate that the embryo is developing abnormally (and not using its yolk?). And a small yolk sac suggests there are not enough nutrients to sustain the developing pregnancy. (I couldn’t find any medical journal articles suggesting that, just a website.)
Okay, this article says in the first trimester a normal yolk sac is 3-6mm and that a yolk sac of less than 3mm is “too small” and “related to spontaneous abortion prediction”:
BUT, later in the article it suggests that at 6 weeks pregnant, the mean is 3.1mm in a “normal” pregnancy with a range of 2.5-3.8mm. Okay, assholes, so if 3.1mm is the mean in a normal pregnancy, how can below 3mm be the beginning of the bad range? Later the authors cite an article (by Green) that suggests that between 8 and 12 weeks pregnant, a yolk sac diameter below 2mm is associated with an adverse pregnancy outcome.
So basically I’m calling bullshit on the suggestion that 3mm is “too small.” The only data the authors cite suggests that 2mm is “too small.” (I could not find a free full copy of the Green article.)
This article suggests that for women of 6-9 weeks pregnant, a yolk sac of 2-5mm is normal:
“The pregnancy outcome was optimum when the sac diameter ranged between 2 to 5 mm. The live pregnancy rate increased to 99.2% (250/252) with this yolk sac diameter range. However when the yolk sac diameter fell outside of this range live pregnancy rates were significantly decreased (for < 2 mm - 50%, > 5 mm – 66.6%).”
In other words, half of the women with “small” yolks (under 2mm) miscarried. 33% of the women with large yolks (above 5mm) miscarried.
The article also explains why a large yolk sac is a problem: “If a large yolk sac persists, it indicates aberrant embryonic development and high chance of miscarriage.” There was no explanation for the cause of a small yolk sac or the reason for adverse outcomes with a small yolk sac.
The article also summarizes a bunch of other studies on embryo heart rate and yolk sac diameter.
The article also suggests that a study by Figueras “found that yolk sac volume outside the 5th to 95th percentile were associated with significant occurrence of retrochorialhaematoma and subsequent pregnancy loss.” (That study’s range was 2-5mm as well.)
Okay, wait, back to the hematoma?!?! Retroplacental hematomas are hematomas entirely behind the placenta and not touching the gestational sac. (As opposed to subchorionic hematomas, which are between the chorion and endometrium.)
Okay, here’s the Figueras article that talks about yolk sacs and hematomas:
For women from 6-10 weeks pregnant, they collected a number of things, including fetal heart rate (FHR), mean gestational sac diameter (GSD), mean yolk sac diameter (YSD), gestational sac volume
(GSV), yolk sac volume (YSV) and presence of a subchorionic hematoma affecting at least 30% of the chorionic plate. The authors found that maternal age (above 34), yolk sac volume, mean gestational sac diameter below than the 5th percentile, gestational sac volume below than the 5th percentile, and fetal heart rate below than the 5th percentile or above the 95thpercentile, were all significantly associated with increased miscarriage. Mean yolk sac diameter and presence of hematoma “showed no significant association with abortion.”
Music to my ears!
Well, let’s not get too excited. They suggest, “There was a tendency toward an association, although not statistically significant, between YSD [yolk sac diameter] and abortion [miscarriage].” It mentioned nothing about hematomas beyond what I mentioned above. Sigh.
Here’s another article that considers 2-5mm to be a “normal” yolk sac diameter for 5-6.5 weeks pregnant. (Although it also notes elsewhere that 3-6mm is “normal”.)
Again, it suggests that for abnormally large yolk sacs, “Larger yolk sac diameters may represent evidence of certain diseases and the pregnancy loss in these pregnancies is reflective of the presence of such underlying diseases.”
But, again, there is no suggestion of why a small sac was a problem. It just suggests, “The lack of a yolk sac or a smaller than gestational age yolk sac diameter are indicative of pregnancies that may result in spontaneous abortion. Pregnancies with a very large yolk sac are generally always associated with poor outcomes”
The source for the small sac diameter reference in that article is here:
This article sets up four groups:
Group I (42nd to 55th day (6–7weeks + 6 days)): pregnancy continued
Group II (56th to 69th day (8–9 weeks + 6 days)): pregnancy continued
Group III (70th to 84th day (10–12 weeks)): pregnancy continued
Group IV (all): pregnancy did not continue
In other words, three of the groups had continued pregnancies and one was the miscarriage group. Then they measure embryo heart rate and yolk sac diameter in the two groups:
Embryonic heart rate and yolk sac diameter in the first trimester
| ||||||
Embryo Heart Rate (bpm)
|
Yolk Size Diameter (mm)
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Group mean (ave)
|
Min-Max
|
Median (middle)
|
Group mean (ave)
|
Min-Max
|
Median (middle)
| |
I
|
129.80
|
96–160
|
132
|
3.97
|
0.00–6.90
|
4.00
|
II
|
142.80
|
120–160
|
140
|
3.98
|
5.00
| |
III
|
146.88
|
132–163
|
140
|
4.22
|
5.00
| |
IV
|
102.33
|
92–116
|
102
|
2.30
|
0.00–3.90
|
2.70
|
The authors conclude, “We demonstrated the decreased diameter of pregnancies with poor outcome and we agree with recent studies that support the negative predictive value of the absence of yolk sac.”
What their numbers show is that the mean (average) yolk sac diameter for the miscarriage group was 2.30mm. The mean yolk sac diameter for the groups where the pregnancies continued was around 4mm and up. (The median (middle) measurement for the miscarriage group was 2.70mm. The median for the groups where the pregnancies continued was almost 4-5.) Note also that the pregnancy loss group had much lower heart rates. Mean (average) of 102 versus 130+. (Median (middle) was similar.)
Sigh. I’m not feeling good about my 2.5mm yolk sac.
Okay, almost done. This article suggests that in week 6, yolk sac diameter mean is 2.9mm (min 2mm, max 7.2mm):
That does not make 2.5mm look too bad. It again concludes that “When compared with pregnancies that had a yolk sac diameter <5 mm, the pregnancies with a yolk sac diameter ≥5 mm had a significantly higher risk of miscarriage.” There was not much about small yolk sacs.
This article suggests “Usually the inner diameter of a yolk sac measures 3 to 5 mm”:
It also has the most detail (but still not a lot) about a small yolk sac:
“Small Yolk Sac
The literature presents scant knowledge about the clinical importance of small yolk sacs. It has been claimed that a very small yolk sac may be a normal finding during early periods of normal embryologic development. On the other hand, a much earlier published study, with certain limitations, suggests that a yolk sac diameter of 2 mm or less may be associated with an adverse outcome in pregnancies with a gestational age of 8 to 12 weeks (Figure 7).14
It is well known that the yolk sac size begins to decrease during the late weeks of the first trimester.1,12 This process is why gestational age should be taken into account when the size of the yolk sac is assessed. However, it would be prudent to perform serial sonographic examinations within a short period whenever a smaller-than-expected yolk sac has been visualized.”
Okay, so a small yolk sac might be okay as long as it’s not below 2mm. I guess only time will tell if my tiny sac is a problem (potentially caused by my hematoma??!!) or not.
Thank you for this extremely educational post. I have been looking everywhere for someone to spell out a small yolk sac and it’s meaning. Thank you. I pray that everything went well with your pregnancy after all this! Please let me know how it went. Thank you again!
ReplyDeleteMama G, I'm so sorry you're going through this! I hope everything works out. Unfortunately my pregnancy did not--after multiple fetal abnormalities were diagnosed at 12 weeks, baby passed away. I can't say if it had anything to do with my small yolk sac, though.
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