Since blood and daily urine excretion of creatinine shows minimal fluctuation, creatinine excretion is useful in determining whether 24-hour urine specimens for other analytes (e.g., protein) have been completely and accurately collected. To view specimen requirements and codes please Select a regional laboratory.
Ultrasound is almost foolproof at diagnosing twins, but the greater the number of babies, the less accurate the ability to detect how many there are. It's possible to see twins (or more) on a scan at around six weeks, though one baby may be missed at this early stage. Sometimes a heartbeat is seen in one sac, but not in the other. Rescanning in a week or two may reveal a second heartbeat, or the scan may show that one sac is growing and the other still empty.
That being said we see a few women have twins appear at 12 weeks scans and very occasionally at the 20 week scan. When my mum was a student mid wife she had a triplet appear at birth. Back then they used xray to establish fetal position for twin natural births. They saw the two babies and went to deliver then BAM a third ass starts appearing.