Beta hcg

    The first day of my last period was Aug 3rd (usually irregular).
    Date of last intercourse was Aug 24th.
    Beta hcg (blood) was Neg. on Sept 2nd.
    Beta hcg was at 120 on Sept 9th and at 1443 on Sept 16th.
    I've been given conflicting estimates as to my due date.
    What would you estimate it as being.
Dear Colleen:
An ESTIMATED Date of Confinement (EDC) is traditionally calculated from the Last Menstrual Period (add 7 days and substract 3 months): Hence, your EDC calculates to May 11, 1999. The calculation ASSUMES all women have a 28 day cycle and ovulation on the 14th cycle day.
The date of last coitus does not help. The date we need to refine the calculation is the day of ovulation (available in patients monitoring their cycle with a Basal Body Temperature record, urine ovulation detection kit, or who have received ovulation induction drugs). The other date we need is the date of conception (available in patients who have timed coitus to ovulation or undergone assisted reproductive techniques).
Since we know the hCG was negative on cycle day 30 (September 2) and positive on cycle day 37 (September 9), the best we can do is to take the midpoint between these two dates and extrapolate backwards. Hence, the assumption causes me to add 3.5 days (round to the next whole day, hence 4 days) to the calculated EDC using LMP.
Finally, my guess is an EDC of May 15, 1999.
A GUESS you exclaim! Yes. By every known method, an EDC is  the mid-point in a range. Less than 10% of patients delivery on their EDC; but, 95% of women deliver in the one month (2 weeks before and 2 weeks after) the calculated EDC. An EDC error does not become important until it is larger than 2 weeks.
A refinement of the EDC can be made using ultrasound measurements. However, the error in these measurements, depending on the point in gestation, is 7-16 days.
This information is provided for general education purposes only and is not a medical consultation. If you have specific questions, please contact your physician.