The Amazing Race....for Sperm

Making a baby is a beautiful process, a miracle of nature and a thing to be cherished. But when you get down to the microscopic level, the journey from insemination to a healthy, beautiful baby is a dramatic saga full of twists and turns.

This is especially true of the Amazing Race of the sperm into the vagina. While millions of sperm enter your body in an average insemination, 99.9% of them don’t stand a chance of advancing past the vagina. For the tiny percentage that do manage to enter the cervix, it’s a race against the clock. Sperm themselves can live as long as 48 to 78 hours. But the fertile period of the egg is even less than that: within a mere twelve hours of ovulation, your egg has lost its peak potency. With such strict constraints, timing is everything.

There’s one particularly sticky obstacle sperm must overcome: cervical mucus. Cervical mucus is a liquid that coats the opening to the uterus, letting active sperm in but keeping bacteria and poor-quality sperm out (hey, a girl’s gotta have standards). If you look at it under a microscope, cervical mucus actually has a mesh-like texture, which changes throughout the month to let sperm through.

This is why if you pay close attention to your discharge, you can often tell what time of the month it is. Several day before and after ovulation, your cervical mucus thickens to resemble a yellow/white cream, and it’s harder for sperm to fight its way through this sticky trap. (Fun fact: this kind of cervical mucus tends to be stringy, so it can actually be cut with scissors.) But when you enter your fertile window and close in on ovulation, your increased estrogen levels increase your volume of cervical mucus by ten times the previous amount, making it thinner and more liquid—kinda like clear uncooked egg whites - making it the perfect texture to let sperm swim through.  

Cervical mucus is the ultimate obstacle for sperm. At first, sperm bounce against the cervical mucus, and if you watch under a microscope it seems like they’ll never get in. Soon, however, a few bold sperm begin to make an indentation in the cervical mucus, and finally—they break through! One by one, the sperm enter the cervix through this small opening, swimming in a line up through the uterus and into the fallopian tube.

So why is the journey to the egg such an obstacle course? After all, you’d think your body would want to make it as easy as possible to get pregnant! Actually, in its own sneaky and brilliant way, this tricky obstacle course does make pregnancy more likely. Because the egg’s window of peak fertility is so short, delaying the sperm’s arrival allows for a steady stream of high-quality sperm for up to a few days, increasing your chances that the perfect sperm will find and fertilize your egg at just the right moment.