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Conception Education

How Nature Chooses Boy or Girl


Sure, you know how to make a baby. But if you're hoping to conceive a girl or a boy with any of the gender selection methods -- high tech or low tech -- you need to understand getting busy with the egg and the sperm, and find out how nature chooses boy or girl.

X-Sperm Make Girls,
Y-Sperm Make Boys

Life begins at the moment of conception, when an egg and sperm unite. There are two kinds of sperm: X and Y. If a X-sperm fertilizes the egg, a baby girl is conceived; if a Y-sperm does the job, a baby boy is conceived. Semen contains virtually equal numbers of X and Y sperm -- yes, even fathers of all boys or all girls have both X and Y sperm!

Ovulation - The Emergence of an Egg

  1. Ovary - The egg comes from here
  2. Fallopian Tube - The egg travels here to meet the sperm and be fertilized
  3. Uterus - The embryo implants here and grows into a baby
  4. Vagina - Sperm get in through here, Baby comes out through here

Each month during a woman's fertile years, an egg is released from her ovaries during ovulation. You have two ovaries, one on each side of your uterus, and each is about the size of the walnut.

When you were born, your ovaries already contained a lifetime supply of eggs. You'll never make any more eggs, but you needn't worry about running out; your ovaries contained about a million eggs at birth, and though that number dwindles to a few hundred thousand viable eggs remaining by puberty, only around 400 to 500 eggs are actually ovulated during the years between puberty and menopause.

Meet the Follicles

At the right time during your cycle, the ovaries receive a hormonal message that menstruation from the previous cycle has ended, and it's time to begin preparing an egg for ovulation.

Inside the ovaries, each waiting egg is encased in its own envelope of cells called a follicle. A number of follicles begin to grow larger, and nurse cells are formed within the follicles to nurture the egg.

At some point, one follicle is somehow selected as the dominant follicle, and it continues to grow and mature, while the others are simply reabsorbed. No one is sure exactly how the dominant follicle is selected, but this follicle "competition" seems to be a way to naturally select the healthiest and most promising egg in a cycle. On some occasions, more than one follicle matures, and fraternal twins (non-identical) can result from the multiple eggs that are released.

Fertility drugs can be used to prevent this competition, and allow many follicles to mature and produce a number of eggs in a single cycle.

But the normal situation is that only a single follicle, on one of the ovaries, matures. It was once thought that the ovaries took turns releasing the egg in alternate cycles (and you'll still hear this from many sources), but it's now known that whether the right or left ovary releases the egg each cycle is fairly random.

Over several days, the follicle continues to grow until it becomes practically larger than the ovary itself, until it is nearly golf-ball-sized. A mature follicle can be felt during a pelvic exam, and is visible by a vaginal ultrasound.

At last, the follicle literally erupts, bursting open and releasing the egg. (I think we can all agree it's for the best that this process happens discreetly hidden away inside the body where we don't need to see it occur.)

The Incredible Oocyte

I'd rather just call an egg an egg, but that'd be too easy for us laypersons so medical science has invented more terms:

  • An oocyte is an unfertilized egg (pronounced oh-oh-site or maybe oh-wuh-site, one of the sillier words in biology)
  • An ovum is an egg after it's been fertilized (plural ova)
  • A gamete or sex cell means either an egg or a sperm.

By any name, the egg is a giantess compared to other cells, in fact it's the largest cell in the human body; at about 100 microns across, it's the size of a small grain of salt, and just barely visible to the naked eye.

Human Oocyte, Actual Size

The egg will need its bulk after fertilization, to support the first cell divisions of the baby-to-be.

Fimbria to the Rescue

The egg, once released by its follicle, must enter the fallopian tube which leads from the ovary to its final destination, the uterus. But inexplicably, the fallopian tube isn't actually attached to the ovary. As the egg exits the follicle, it must be quickly rescued from tumbling into the pelvic cavity by the fimbria, fingerlike projections at the end of the fallopian tube. The fimbria draw the egg into the waiting fallopian tube, and it is here that the egg awaits to be fertilized by sperm.

But the egg's hours are numbered. After literally waiting a lifetime for this moment, the oocyte can survive only 24 hours after ovulation. This is why it is so critical to detect when ovulation occurs, if you are trying to conceive. However, the saying that "you can only get pregnant one day of the month" isn't true, because sperm can survive up to 5 days, under the right conditions; you could have intercourse on Monday, and conceive on Friday. But your best chance of getting pregnant is having intercourse the same day as ovulation.

The egg, the largest human cell, is unique in another way. Every other cell in your body contains your entire genome in 46 chromosomes, but an oocyte contains only half that number, destined to combine with a matching set carried by a sperm.