In vivo artificial insemination may be performed one or two time in a treatment cycle. It may depend on the clinical situation. There are several published studies discussing this issue.
Some studies show no improvement in pregnancy rate with two inseminations on sequential days (or 24-36 hours apart) as compared to one well-timed insemination. Other studies show significantly higher pregnancy rates when two inseminations are performed.
A possible explanation for this discrepancy might be that if the single insemination is not properly timed with respect to ovulation, pregnancy rate should improve when the protocol of two insemination in a treatment cycle provides at least one well-timed insemination.
Any insemination should be carefully timed and performed at or a little before the expected time of ovulation, because the oocyte generally survives for 12 hours only if it is not fertilized with a spermatozoon.
It is well-known that sperm may remain viable in the female reproductive system for five or more days and then is still able to fertilize an egg.
Since the golden time for egg being fertilizable is 12 hours after ovulation, artificial insemination performed 24 hours after ovulation is less likely to result in fertilization and pregnancy.
Since two inseminations per cycle might result in some improvement in pregnancy rate, the additional cost and work could be worthwhile.
Some infertility clinics perform 2 inseminations per cycle, some clinics do one insemination. Generally we perform one insemination; in some situations we will encourage the couple to do two as there is no significant disadvantage by doing two, particularly we use the Wang Tube System, which is superb, ideal and ultimate in sperm separation (or selection) for either in vivo or In vitro fertilization.