Pregnancy and periods

Dear Eva,
My friend told me that a girl can't get pregnant if she has sex on her period. Is this true? I also heard a girl can get pregnant before her first period!

To answer your questions, we need to take a look at how the menstrual cycle works and what happens before the first menstrual cycle.

The menstrual cycle is a phase that recurs often in an adult woman's life, lasting anywhere from 24-36 days on average. This doesn't mean that she's on her period for the duration of the menstrual cycle! No, the female body is busy going through different hormonal changes at different points in the cycle, only one of which is your actual period.

You start counting your cycle from the first day of your period, Day 1. When an egg is not fertilized, your body knows it's safe to shed your uterine lining along with the unfertilized egg. It also cues your ovaries to start working on the maturation of a new egg, using a hormone called FSH -- Follicle Stimulating Hormone. Aptly named, huh?

Because you have numerous egg follicles, about 20 of them will be soaking up all that FSH, but one particularly greedy egg starts to hog it all. As this egg becomes more mature, it begins to secrete its own hormone called estradiol. Estradiol is a type of estrogen which tells your body, "Hey! This egg is ready to go!" so that you'll stop producing FSH.

Over the next week or two, your uterine lining starts to build back up in preparation for the new egg.

About two weeks after the beginning of the cycle, the body's accumulated estrogen causes the pituitary gland (remember that from sex ed?) to produce lutenizing hormone; this hormone causes the follicle to release the egg. This is known as ovulation, and it's the time during which a woman is most likely to get pregnant.

The follicle that released the egg becomes known as the corpus luteum, as it withers and starts to collect fatty acids around itself. The corpus luteum secretes a substance called progesterone, which prevents the egg follicles from developing, while your uterine lining continues to prepare for the implantation of the new egg.

If the egg is not fertilized and/or does not implant, your period begins again and the whole cycle starts over. This is some real Circle of Life stuff!

If you didn't catch all that you can see a fascinating animation here.

So does this mean that a woman can get pregnant on her period if she has unprotected sex?

Yes! Yes, yes, yes! Although the released egg is only able to be fertilized for about a 24-hour period, it is often difficult to predict when ovulation will occur. Additionally, sperm can live up to 7 days inside a woman’s vagina. If you had sex during your period and the sperm continued to live inside the vagina, they could potentially travel up to meet with a newly-released egg.

What about that first period thing?

This was a fascinating topic to research. Doctors and scientists aren't 100% certain what actually causes menarche (a girl's very first period), but they do know that it is triggered in a similar way to other periods. A wave of LH and a smaller release of FSH propel the menstrual cycle into motion -- but this initial hormone surge may not be sufficient to cause ovulation. A cycle in which a woman does not ovulate is called anovulatory; a cycle in which an egg is produced is ovulatory.

A study conducted by Department of Obstetrics and Gynecology at the Medical University of Debrecen in Hungary involved collecting urine samples from 51 girls over a 2-year period to determine how often ovulation occurred. They said:
The frequency of ovulatory cycles was 10-13% after the 1st 7-10 menstrual cycles; it approached 50% after the 20th cycle.

So you can see that it takes some time after the first menstrual cycle for ovulation to become a regular event for most girls -- but that doesn't mean it's true for every girl. That initial rush of LH and FSH could potentially prompt ovulation before the first period, and subsequent periods may be ovulatory even early on.

Because it is almost impossible to predict, a girl should always practice safer sex no matter where she is in her menstrual cycle, and use barrier methods of birth control (such as condoms) to help prevent pregnancy.

If you have been menstruating for about 2 years, this is a good time to talk to your doctor about the option of hormonal birth control. I do not recommend birth control for girls who have not yet had about 20 menstrual cycles, but if you have concerns about pregnancy or your period, talk to your doctor.

In short: A girl can become pregnant on her period, and a girl can become pregnant before her first period, although it is unlikely. Don't take any chances, always use a condom!

- Eva

PS. If you want more information about the female reproductive cycle and how it works, head down to your local library! The librarians at the reference and circulation desks will be happy to point you in the right direction.