Theoretically it should be equal to an Y-chromosome-carrying sperm being the first to fertilise the ovum as many have suggested.
Nevertheless, this is contrary to facts of demography where observations of human populations have shown that the natural sex ratio is approximately 105 males to 100 females at birth, which reflects a disproportion in how the natural probability is realised.
""Humans have a Fisherian sex ratio. In humans the secondary sex ratio is commonly assumed to be 105 boys to 100 girls (which sometimes is shortened to "a ratio of 105")."
Four stages, also theoretical, are supposed to affect the sex ratio, the first of which is what you have in mind.
"Types of sex ratio
The sex ratio varies according to the age profile of the population. It is generally divided into four:
primary sex ratio -- ratio at fertilisation
secondary sex ratio -- ratio at birth
tertiary sex ratio -- ratio in sexually active organisms
quaternary sex ratio -- ratio in post-reproductive organisms
Measuring these is a problem since there are no clear boundaries between them."
If we disregard the ratio of natural premature termination of an embryo or a fetus to be equal between the sexes (which the next poster rejects as unsicientific, so only for the sake of arguement), then the secondary ratio can be used to reflect the primary ratio.
The going secondary sex ratio being 105 (Y) to 100 (X), a primary sex ratio of the same value gives 100 / 205 = approx. 0.439.
If the assumption has any quantitative validity, then the chances that a sperm carrying an X chromosome to fertilize the ovum first would be close to 0.439. (If the probability of a premature termination of an XY carrier is greater than that of an XX carrier, then the actual probability would be somewhat less than 0.439.)
http://en.wikipedia.org/wiki/Gender_imbalance