Conception date and ultrasound accuracy


Q: Hello,

I believe that I am 14 weeks 4 days pregnant. Due 2/26. I stopped birth control in late February. I have since had a period on March 15, April 9th, May 1, and May 25. The first day of my last period was May 25th. I had it for 5 days and had a very heavy flow. I believe we conceived on June 6th. I took a home pregnancy test on June 16th and it was positive. I went to my doctor on June 18th and the urine pregnancy test was negative and the blood test showed beta hcg level of 54 and 13.1 progesterone. The follow up blood test showed hcg level of 804 and progesterone of 17.1 on June 24th. On July 20th we had a ultrasound and was told the baby was measuring 8 weeks exactly. The heartbeat was 166.

I have read online that ultrasounds can be incorrect. Can they be off by 8 weeks? Could the baby be measuring small? Is there any way the baby was conceived in April or May? How accurate is my due date based on the information provided and ultrasound?

Symptoms:  June 20- August 8th extreme fatigue, tender breasts, increased discharge
Conception date and ultraso...-1

A:   Hello,
Thanks for the query at ATD about your health care.

I have gone through your history and in my opinion ultrasound scan is a best method to calculate due date,baby size and weeks of pregnancy.

There is a difference of 2 weeks in due date calculation from ultrasound or LMP because of conception may occur early or late.

In ultrasound if it done under expert hand chances of 8 weeks difference is not possible ( but it should be done early) .

Yes,it is possible that baby is small for date so the parameters are low and calculation differ.
But 8 weeks difference is not possible.

Possibility of conception in your case after 25 th may and around 9th- 10 June.

Hope found useful.
Good luck.

Comments / Follow Ups

Patient: Hello,

I am confused by your response. There was only a 3 day difference in my ultrasound and lmp which to me is understandable. I also understand you are saying under an expert the 8 week difference is not possible. Is any difference over 2 weeks possible? I am sure about my period dates. Is it 100% not possible to have conceived in early April?
Doctor: Hi,
Sorry you should not be confused.
One important thing that 8 weeks difference is not possible.
Yes in ultrasound scan due date may vary from days because we calculate our due date with the help of last menstrual period but in ultrasound scan it can calculate it since conception which can be 10-14 days prior to your missed period date.
In my opinion as you had proper period on 25 th may than the chances of conception before it is not possible.
Hope it is clear now.
If you need further clarification I am happy to help you.
Good luck.
Patient: I have heard some women have regular periods during their pregnancy. Could this be a possibility in my case? All 3 were very heavy. Also, how do my hcg levels correspond to the dating of the pregnancy? I feel like the chances are very slim of being 8 weeks pregnant and only having a beta hcg 54. Is this even possible?
Doctor: Hi,
Yes,it is true that women are pregnant with normal menstrual cycle.
So in your case it can be a possibility but as you have very heavy menses,the chances are quite low.
In some cases initial level of hcg can be low but it doubles infai
Doctor: In some cases hcg can be low initially but double 24 hrs.maybe in your case it is like that.
Hope it is clear now.
Good luck.
Patient: Ok just to be clear. I should follow the ultrasound dating as the most accurate since this also corresponds with my lmp.All other possibilities are low?
Doctor: Hi,
Yes,you can rely on ultrasound scan.
Good luck.

Related Q&A

Q: What to take if you dont want to be pregnant and your period hasn't come for a month

Q: I rubbed my penis over my girlfriend's vagina. I did not ejaculate. It was covered in pre-ejaculate though and I had ...

Q: Hi I'm a 17 year old female and two weeks ago I started taking the contraceptive pill. Unfortunately I wasn't infor...

Q: If I my girlfriend and I were grinding naked during ovulation or the day before and I had Precum on my finger and fin...

Q: Had sex on day 14/15 of my cycle which is usually 27/28 days. We used a condom but it broke . He wasn't near ejacula...


Dr. James Obaji M.D.

  • 247 Reviews
  • Family Physician

Dr. Suneel Sharman M.D.

  • 190 Reviews
  • Family Physician

Dr. Pat Golden M.D.

  • 169 Reviews
  • Cardiologist

Dr. Kunaal Jindal M.D.

  • 112 Reviews
  • Plastic Surgeon