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Is missing a gestational sac during first ultrasound possible?

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Posted on Fri, 20 May 2016
Question: I found out I was pregnant via home pregnancy test when I missed my period but was having lots of menstrual cramps - test said 2-3 weeks pregnant (which is the timeframe I was thinking). I went to urgent care the next morning for the cramping - they did an intravaginal u/s and they said they could not see anything in the uterine cavity, but thought I may have a corneal ectopic pregnancy. My hcg level was 551. Two days later, another u/s still showed questionable corneal ectopic, but they thought that they saw a 'small gestational sac' (I'm unsure of the size), so they were leaning towards not ectopic. Hcg was 1057. Two days later, hcg was 2766, and they ruled out corneal ectopic. However, they said that the 3rd u/s showed that my sac is 44mm (with probable yolk sac, and no fetal pole). I also have a collapsed cyst on my left ovary. They told me it will probably be a blighted ovum, and I need to do a viability scan in two weeks. I am confused, because I thought the sac grew approx 1mm/day. So how come they could not see the sac on the first ultrasound, see apparently a small one two days later, and then two days later see a 44mm one? Am I wrong and I am possibly 9/10 weeks pregnant (instead of approx 5 weeks), or is it possible there is a mistake in the measurement of 44mm???
i am worried about how big the sac is, because I know that 44mm with no fetal pole will be a blighted ovum. But is it possible to miss such a large sac on the first u/s? Also, I'm concerned that the cyst is collapsed - will that contribute to miscarriage? Any help would be appreciated, because I cannot wait 2 more weeks for another scan wondering about this.
doctor
Answered by Dr. Deepti Verma (16 minutes later)
Brief Answer:
Blighted ovum likely

Detailed Answer:
Hi dear,
I have gone through your question and understand your concerns.
Gestation sac is visible only when the hCG levels are around 1500, therefore it may be possible that the gestation sac was not visible on the first ultrasound.
It is rare to calculate the gestation sac wrongly by a trained ultrasonologist.
If the fetal pole is not visible at this gestation sac size of 44 mm, then it is most probably the blighted ovum.
Collapsed cyst can be corpus luteal cyst, and it is not at all the cause of blighted ovum.
Chromosomal anomalies are usually the cause of blighted ovum, and these generally have no recurrence rate in subsequent pregnancies.
Hope you found the answer helpful.
Regards
Dr Deepti Verma
OBGYN
Maternal and fetal medicine specialist
Above answer was peer-reviewed by : Dr. Naveen Kumar
doctor
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Follow up: Dr. Deepti Verma (7 minutes later)
Hi Dr. XXXXXXX

Thanks for your answer. I totally understand what you are saying about the sac not being visible until about levels of 1500. I also understand if the sac is empty at 44mm, it is most likely a blighted ovum.

However, can a sac grow that fast - ie. Friday - not visible, then following Tuesday (4 days later) at 44mm? I thought that the sac grows approx 1mm/day?

Also, if my timing is right, the Tuesday ultrasound (where they say my sac is 44mm), I would only be 4 weeks, 6 days since LMP. Is that even possible to have a sac at a size of 44mm this early?

I think my confusion lies in how fast the sac apparently grew, and how much it grew in the timeframe that I believe I conceived. Could you shed some more light on that?

Thanks so much.
doctor
Answered by Dr. Deepti Verma (8 minutes later)
Brief Answer:
Please get a repeat ultrasound

Detailed Answer:
Hi dear,
You are correct in saying that the gestation sac do not grow more than1.5 mm a day.
Moreover, a sac size of 44mm is not possible at 5 weeks of gestation.
There has been some mistake in ultrasound reporting.
For confirmation, you can get a repeat ultrasound from a different centre, if possible.
Regards
Dr Deepti Verma
Above answer was peer-reviewed by : Dr. Naveen Kumar
doctor
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Follow up: Dr. Deepti Verma (19 minutes later)
Would 4.4mm be more typical of an almost 5 week gestation?
doctor
Answered by Dr. Deepti Verma (10 hours later)
Brief Answer:
Yes

Detailed Answer:
Hi dear yes, the size of g sac will be around 5 mm at this gestation.
Regards
Dr Deepti Verma
Above answer was peer-reviewed by : Dr. Prasad
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Dr. Deepti Verma

OBGYN, Maternal and Fetal Medicine

Practicing since :2009

Answered : 5068 Questions

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Is missing a gestational sac during first ultrasound possible?

Brief Answer: Blighted ovum likely Detailed Answer: Hi dear, I have gone through your question and understand your concerns. Gestation sac is visible only when the hCG levels are around 1500, therefore it may be possible that the gestation sac was not visible on the first ultrasound. It is rare to calculate the gestation sac wrongly by a trained ultrasonologist. If the fetal pole is not visible at this gestation sac size of 44 mm, then it is most probably the blighted ovum. Collapsed cyst can be corpus luteal cyst, and it is not at all the cause of blighted ovum. Chromosomal anomalies are usually the cause of blighted ovum, and these generally have no recurrence rate in subsequent pregnancies. Hope you found the answer helpful. Regards Dr Deepti Verma OBGYN Maternal and fetal medicine specialist