13.5 week gender ultrasound

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We went to an independent ultrasound place a few weeks ago and they said it was a girl. I showed it to my dr yesterday and she said it looked like a girl but since it was done before 16 weeks she wouldn't believe it til I have the 20 week scan. I find out for sure the 31st at 20 weeks but wondered what you guys think. Definitely girl or possibly not totally developed boy? Anyone else had a early gender scan be wrong?