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Wrong dates from ovulation cycle and by ULTRASOUND.

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Hi doc

I had my ultrasound done on July 3, which say 8 week 1 day old. My first date of my last menstrual period is April 24, 2015.
I am at a 35 day cycle.
Also on May 13 - 16, I have blood spotting.

1) for my ultrasound on jul 3 2015, it dated 8w1d. it is the gestational age. But it is actually measured by the fetus size. So is it really a gestational age? Please confirm.

2) per my blood test did on June 9, I was told that I was at my 4th week. So my doc was very confirmed that I must have conceived in the week of May 10-16. Is that true?

3) I understand that the Calculation of expected due date is done by Naegle's formula, and it is applied assuming that the lady has a normal28 day cycle, then add 9 months and 7 days to last menstrual period to calculate the due date.
I have prolonged menstrual cycles (35 day) then the due date is calculated by the first trimester ultrasound. Whatever the fetal age comes, assume it to be gestation age and retrospective calculate the last menstrual period according to it, and then apply Naegle' s formula to calculate the the due date. So in this case, if I am 8w1d (may 7 2015) per ultrasound, though it is measured by fetal age, we still assume it to be gestational age and treat it as LMP and calculate with Naegle's formula? Is my understanding correct? So would I have a due date of January 31,2016 base on LMP on April 24, 2015 or due date of Feb 11, 2016 based on May 7, 2016.

4) with the above info, what would be by due date and conception date?

Many thanks

Category: OBGYN

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Category: Infertility Specialist
Dr. Aarti Vazirani is online now

Expert:  Dr. Aarti Vazirani replied 4 Days.

Thanks for posting your query at DoctorSpring.com
Let me clarify your doubts -
1. Yes the ultrasound on july 3rd is the gestational age. It is calculated from fetal size, but it is the gestational age as most algorithms calculate the gestational age directly from the size of the fetus.
2. Based on the scan, it is very probable that you conceived in the second week of May, I would say after 10th of May, more probably around the 20th - 22nd of May. Blood tests cannot be used to date a pregnancy, they are not reliable.
3. Negele formula and EDD based on scan are 2 different things. Negele formula is for a woman with 28 day cycles. For a woman with 35 day cycles, we just add 7 more days to the EDD. So if your LMP was 24th April, your EDD would ideally be 31st January 2016, and 7th of february , 2016, by adding 7 more days for a 35 day cycle.
Going by your ultrasound scan, your EDD would be 14th of February ( LMP - 7th of May, and then 9 months and 7 days )
4. I would say you conceived around the 22nd of May, and your due date would be 14th of February, as in case of delayed cycles, the first ultrasound scan is the most reliable indicator and the scan dates are considered final.
Take care.
Feel free to discuss further.

Dr. Aarti Vazirani
Category: Infertility Specialist
Residency: Obstetrics & Gynecolog, BJMC,Civil hospital, Ahmedabad, 2008
Post Graduate: MS (Obstetrics & Gynecology), B.J. Medical College, Ahmedabad, 2008
Medical School: MBBS, B.J. Medical College, Ahmedabad, 2004
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