Confusion about ovulation timing: early beta hcg vs ultrasound dating

Hi everyone,

I’m looking for objective opinions about pregnancy dating based on beta-hCG values and early ultrasounds.

Background:

• Last menstrual period: November 20

• Cycles before pregnancy: , about 32 days

• I had stopped hormonal birth control months earlier and had regular cycles before conceiving

Beta-hCG results:

• December 21: beta-hCG = 16 mIU/mL

• December 24: beta-hCG = 99 mIU/mL

Early ultrasound:

• January 21: embryo measured 8.7 mm (CRL)

• Cardiac activity present

• Based on this ultrasound, my doctor estimated ovulation around December 16 or 17 and set the due date as August 27

What feels confusing to me:

If ovulation really occurred around December 16 or 17, then my first positive beta-hCG on December 21 would have been only 3 - 4–days post ovulation

From what I understand, at 4–6 DPO implantation usually has not happened yet, and beta-hCG is typically not detectable in blood at that stage.

This is why I’m having trouble reconciling:

• the ovulation date suggested by my doctor

• with the early beta-hCG results

• and basic implantation timing

My questions:

• What is the earliest and latest realistic ovulation window that would still fit these beta-hCG values and the CRL of 8.7 mm on January 21?

• Is it biologically plausible for a detectable beta-hCG (16 mIU/mL) at only 4–5 DPO, or does that strongly suggest ovulation happened earlier?

I’m not concerned about the health of the pregnancy — everything is progressing normally — I’m mainly trying to understand the timeline from a medical and physiological perspective.

Any insight from those familiar with beta-hCG dynamics or early ultrasound dating would be greatly appreciated.