(Theresno authoritative number for the corresponding rate in men.)

So far, so good.

The time in between periods isnt just a three-week lull, but a hormone-driven dance of biology.

That only happens because because something really interesting is going on inside.

So thats where we synchronize our watches.

Actually,several follicles begin to grow, but only the fastest growing one gets to be ovulated.

That follicle releases its ovum somewhere around day 14 in a 28-day cycle.

If you have a longer-than-usual cycle, itsprobably the follicular phase that is the long part.

Ovulationhappens when the follicle ruptures and the little egg is free to float down thefallopian tubesinto theuterus.

Sometimes there iscramping or spotting, but its also totally normal to have no idea when youre ovulating.

The next 14 days or so are theluteal phase.

PMS symptoms can even feelconfusingly similar to early pregnancy symptomssometimes.

Weve glossed over the hormone surges that orchestrate this whole crazy dance.

Once a womans period has started, PMS is over and shes back into the follicular phase again.

In fact, the likelihood of getting pregnant on your or your partners period isjust about zero.

Its an especially bad idea for teenagers whocant count on their menstrual cycles being the same length every time.

On the other hand, understanding where you are in your cycle can help women understand their body.

If you only watch the calendar, this is the not-very-effective rhythm method you were probably warned against.

Lets zoom in a little to the day ovulation happens.

Thisstudy published in Human Reproductiongives day-by-day probabilities of how likely a woman is to get pregnant each day.

In the week of their period, almost nobody is fertile.

Fertility awarenesscanwork as birth control, but there are some huge caveats.

And third, fertility awareness methodshave a high failure rate.

Keeping track means more than just watching the calendar.

Cervical mucus, which you might otherwise know as vaginal discharge, changes consistency.

When youre fertile, its thick and gooey.

Ovulation test kitswill give a positive result a day or two before ovulation.

Tracking these things can get complicated.

Remember that even with a lot of data its tricky to use this understanding toavoidpregnancy.

Fertility awareness is much more useful, instead, when youre trying to get pregnant.

But on the other end, its even weirder.

Heres your mind blowing fact of the day: Theres no such thing as one week pregnant.

Hereswhat the timeline actually looks likefor somebody with 28-day cycles:

Day 1of your pregnancy: Your period starts.

You are very NOT pregnant.

Day 14: Ovulation.

Egg finally meets sperm.

The fertilized egg divides, and divides again, and so on.

Day 28: The implanted embryo has now secreted enough of thehCGhormone that it can be detected in urine.

You canfinally pee on a stickand see two lines.

Your morning sickness is on its way and will bearriving in about two more weeks.

You may also be able to avoid thewhat if youre pregnant hassle around x-raysand the like.

So thats when babies are made.

Better yet, make use of any ofthese fabulous methods of birth controlthey work any time.

Illustration by Fruzsina Kuhari.