If I’ve learned nothing else in my various debates about abortion, it’s that many people have absolutely no idea where babies come from, beyond the sperm plus egg basics. At the extremes are people who say that birth control pills work by causing abortions, but many who are not spreading misinformation — whether pro-choice or pro-life — seem to have at best a shaky understanding of fertility.
So, since it’s important to anyone wanting to achieve pregnancy and interesting to anyone who cares about reproduction at all, I want to give a bit more background on a woman’s reproductive cycle.
The menstrual cycle can be divided into two parts: the part before ovulation and the part after. Pregnancy can only be achieved for in the 3 to 5 days preceding and including the day of ovulation (more on that later). Once ovulation has occurred without the egg being fertilized, pregnancy cannot occur.
But that’s jumping ahead of ourselves. First, a hormone called Follicle Stimulating Hormone (FSH) causes some eggs to mature in each ovary. Eggs are encased on follicles, hence the name of the hormone. Once one follicle is large enough, your estrogen levels trigger the release of Luteinizing Hormone (LH), which causes the winning egg to be released. The rest disintegrate.
The egg that makes it moves into the fallopian tubes, which connect the ovaries to the uterus[1]. The follicle which formerly held the egg collapses and starts to release another hormone, progesterone. Progesterone prevents other eggs from being released and prepares the uterus for pregnancy (although sometimes more than one egg is released, which, if fertilized, results in fraternal twins). The 12 – 16 after ovulation are called the luteal phase after the corpus luteum, the name for the collapsed follicle.
If fertilization does not occur, the egg dies, and eventually the uterus disintegrates, resulting in your period. However, if it is fertilized, after about a week, it will implant in the uterus. Although numbers are hard to get, ultrasound studies indicate that 50% (or more) of fertilized eggs fail to implant[2]. However, if the fertilized egg, now a multi-cell embryo, does implant, it will cause Human Chorionic Gonadotropin (HCG) to be released. This is the hormone that home pregnancy tests detect, hence why are not not effective until 10 days or more after ovulation — about a week for implantation plus several more details for hormone levels to build up enough to be detectable in your urine.
That’s what’s going on behind the scenes. In a later post, I’ll explore the observable changes caused by all these hormones, which are valuable for a woman trying to achieve (and to a significant but lesser degree, avoid) pregnancy.
Primary source: Taking Charge of Your Fertility by Toni Weschler
[1] Interesting factoid: There is actually a gap between the ovaries and their respective fallopian tubes. This gap serves no purpose — in fact, if a fertilized egg falls into that gap, it can be fatal for the woman. This gap is taken as a bit of evidence in favor of evolution since it’s useless but can be cleanly explained by looking at the fallopian tubes as an addition to a reproductive system that laid eggs directly in the water. From Why Evolution is True by Jerry Coyne.
[2] Libby Anne over at Love, Joy, Feminism has a post that discusses what this statistic means for those who think the pill is bad because it causes abortions: How I Lost Faith in the Pro-Life Movement