… In An Eggshell …
What you should know about your eggs and how
your age affects your monthly chance of having a baby naturally.
About the egg.
The egg is the largest cell by diameter in the human body and perhaps the most remarkable. It is responsible for creating an embryo that will hopefully grow into a baby. True, the sperm has the important role of fertilization, but it is the DNA of the egg that is the driving force behind the first few days of embryo development, during which time the sperm genes remain inactive.
In the months leading up to ovulation, the egg gathers all of the proteins, DNA and energy-producing powerplants (called mitochondria) needed to orchestrate the creation of an embryo. The egg must segregate and duplicate chromosomes over and over as they go from a single cell in the unfertilized state to an embryo composed of hundreds of cells.
The precision and incredible amount of energy needed to create a chromosomally normal embryo that grows into a baby is remarkable!
If the egg has less energy and fails at it’s mission, the embryo will die in the fallopian tube and fail to create a pregnancy or will be miscarried by the 12th week of pregnancy.
Thus, as women age the rate of infertility and miscarriage increase leading to a decrease in live births. This becomes more prevalent after the age of 35, which is referred to as advanced maternal age.
What do your Fertility Calculator numbers mean?
Monthly Birth Rate Chance
It may be surprising to learn that the highest monthly chance of having a live birth is only 20%, which is the rate seen in women under the age of 30. That means 80% of the time women under 30 having regular unprotected intercourse will either fail to conceive or undergo a miscarriage.
It turns out that the human egg is prone to making errors in segregating chromosomes at all ages.
Sometimes women fail to conceive due to factors unrelated to the egg such as sperm factors or blocked fallopian tubes, but the egg factor and specifically chromosome segregation errors still account for the majority of failed attempts.
The monthly chance of having a live birth drops to 15% at age 35, 10% at age 40 and 1% at age 45. Some of this decline is due to loss of egg inventory (or egg count) as women age, but it should be noted that although women lose 88% of their eggs by age 30, the monthly chance of having a live birth only decreases by 1% (from 20 to 19%). So, loss of egg inventory is only part of the story.
Something changes once a woman enters her thirties. It is not a stretch to imagine that any cell remaining in the body for 30 or 40 years may run out of steam and not work as well…especially when it has a job as complex and energy intensive as creating a baby!
We use the term egg quality to describe an egg’s ability to create a chromosomally normal embryo. As women age, egg quality declines and the rate of infertility and miscarriage increases leading to even lower live birth rates.
Are there other ways to estimate the quality of your eggs? The answer is no, not at the current time. However, we know that age is a good indicator of egg quality with live birth rates declining over age in a predictable manner for the majority of women.
Egg Count
Women are born with a set number of eggs that is established during fetal life when they themselves are still in the womb. The eggs are produced during fetal development and peak around the 20th week of pregnancy when as many as 5 million eggs may be present in the ovaries.
Thereafter, genetic signals in the ovary cause waves of eggs to begin growing each month. By the time puberty is reached at age 13 only 350,000 eggs remain. Monthly loss of eggs continues until menopause at which point less than 1000 eggs remain. Nothing, not even pregnancy or birth control, can slow the monthly recruitment and loss of eggs. Even though hundreds of eggs are recruited each month, more than 99% will ultimately die without ever being ovulated. It is estimated that only 450 eggs will be ovulated by a woman during her reproductive years. Unlike men, who produce new sperm every 90 days, women do not replenish their egg supply.
As a woman ages, the eggs undergo degenerative changes, just like any other cell in the body, and the energy-producing powerhouses of the egg (the mitochondria) also diminish.
Average, Low, High Egg Counts: The wide range of egg counts reflects different starting egg counts during fetal life.
Ovarian Testing
Are there ways to estimate the number of remaining eggs in your ovaries, what is commonly referred to as the ovarian reserve? The answer is maybe. Fertility specialists have developed several screening tests to determine ovarian reserve. The most common are blood tests for the hormones FSH (follicle stimulating hormone), estradiol (one of the estrogens), and anti-mullerian hormone.
It should be noted that eggs are each housed in a protective sac composed of hormone producing cells called the egg follicle.
FSH is a hormone made by the pituitary gland that stimulates the egg follicles (sacs containing the eggs) to grow every month. The egg follicles in turn make estradiol which turns off the FSH production so that only one egg is recruited and ovulated per month. High FSH values during the first few days of the period suggest diminished ovarian reserve.
Anti-mullerian hormone is made by granulosa cells which make up the egg follicle (egg sac). It can be measured any time during the month. Values under 1.0 suggest diminished ovarian reserve.
The other common test for ovarian reserve involves visualizing the ovaries with an ultrasound during the first few days of the menstrual cycle and measuring the visible egg follicles. Even though hundreds of egg follicles may begin to grow each month only a limited number will reach a size visible by ultrasound. The number of visible egg follicles is called the antral follicle count. Antral follicle counts less than 8 between both ovaries is considered a sign of diminished ovarian reserve.
These tests should be used carefully as none perfectly identify all women who will fail to conceive. That is to say, women with normal ovarian reserve testing fail to conceive and some with abnormal testing successfully conceive. In addition, the ability of these tests to predict diminished ovarian reserve drops when used in women at low risk for diminished ovarian reserve.
What can you do?
The most important thing women can do to retain their fertility options is to educate themselves on their fertility life cycle. The Fertility Clock and Fertility Calculator were developed to provide women with a tool to learn more about age-related changes in fertility.
The Fertility Clock and Fertility Calculator
We know from landmark studies of historical populations dating back to the 17th century that natural birth rates consistently decline with age. These natural birth rates have been incorporated into the Fertility Clock beginning with a maximum monthly birth rate of 20%, the rate seen in women under the age of thirty. By entering your birth date into the Fertility Calculator you will see the estimated monthly birth rate for your particular age.
It should be noted that pregnancy rates are always higher than live birth rates since pregnancies can be miscarried before birth, hence our preference to use live birth rates.
The Fertility Clock also displays the total number of eggs remaining in both ovaries at any given age. This data is based on actual microscopic examination of ovaries across all relevant ages. The Fertility Calculator also provides the high and low range for egg counts in addition to the average for any given age. The wide range of egg counts reflects different starting egg counts during fetal life.
The Fertility Clock and Fertility Calculator provide an estimate of natural fertility based on the best available scientific evidence. As with all biological systems, some individuals will fall outside the averages presented here, for example those undergoing early menopause. In addition, other fertility factors such as STDs, endometriosis and sperm factors, and general health and medical conditions may greatly impact the chance for a pregnancy and live birth. Women who have concerns about their fertility should seek expert medical advice.