I have been trying to figure out exactly when we conceived. We had sex on 6/7/06 and 6/16/06-my period was due on 6/18-and I didn't get it so I tested on 6/24 and it was positive. My last period began on 5/23/06 and that should mean that I am almost 8 weeks. Is this correct? When I went in for an ultrasound last week, the Dr. only saw a sac and measured it to be a 5 week pregnancy. She said to come back in 2 weeks and hopefully there will be a fetus/heartbeat. Could this mean that I am indeed almost 8 weeks pregnant and that the fetus stopped developing at 5 weeks? I am so confused and worried. Any feedback from you is helpful. Thanks.

The situation of incorrect dates based on the last menstrual period versus an unhealthy pregnancy is always a tough one for the patient and the doctor.

If one has a very regular cycle, ovulation should occur about 14 days prior to the onset of the next period, or a quick way to figure it out is : length of cycle - 14 = day of ovulation. However, on any given cycle one may ovulate atypically early or late and that can make the sonographic appearance difficult to interpret, especially if the ovulation occurred later than expected. Travel, stress, anxiety and change in exercise levels are just a few things that can throw ovulation off.

With a last menstrual period of May 23rd and assuming a 28 day cycle, one would expect ovulation to occur around June 5th or so. Sperm survive between 2 and 4 days after intercourse occurs so one could conceive as late as June 10th if intercourse was on the 6th, and as late as June 20th if intercourse was on June 16th. If one conceived on June 20th however, it would probably not turn the home pregnancy test positive by the 24th. Usually the home kits register a pregnancy about 12 - 14 days after conception. If the test was positive around June 24th, that suggests a conception around June 10th give or take a few days. If conception occurred on the 16th, the test could turn positive by the 24th if the hormone levels were very high, but this is less likely.

If one conceived between June 16th and June 20th that would correspond to a 6 to 6 1/2 week pregnancy and one would expect to see a gestational sac, a yolk sac and a small fetal pole on the ultrasound by July 18th (or a 5 to 5 1/2 week gestation if the ultrasound was last week: gestational sac and very early yolk sac).

It is understandably worrisome when the ultrasound appearance and the dates seem mismatched, but waiting one to two weeks is always the best way to assess what is happening. If there is appropriate growth on the ultrasound in one to two weeks that suggests "wrong dates" and a late ovulation. If the pregnancy is unhealthy and stopped growing a few weeks ago, the ultrasound appearance will not change during the follow up period.

One last thing one can do to try and sort out the difference is to send a blood Progesterone level. This is not by any means exact, but generally speaking levels over 15 are suggestive of healthy pregnancies, levels under 10 are concerning and are more likely to be either unhealthy or ectopic (implanted in the fallopian tube) and levels between 10 and 15 are a grey zone. This level can help your doctor counsel you about the pregnancy but the most important issue is, when in doubt, WAIT. Time is the best way to figure out what is going on with the pregnancy, even though the wait is frustrating and anxiety provoking.

Answered by Dr. Lynn Friedman, M.D.