This is part 1 of an in depth explanation of menstrual cycles, conception and how it relates to gestational age.
Written by Options Nurse Manager- Louise Hall, RN
As a nurse, I have been asked by multiple women what the “conception date” of their baby is. This can be a confusing topic, especially if a pregnancy is dated at, say, six weeks, but she only reports having sex once about four and a half weeks ago. So how can one determine when a baby was conceived?
First, we need to review a few things about a woman’s menstrual cycle. Over a period of about 21-35 days, the hormones in a woman’s body fluctuate. The menstrual cycle begins on the first day of her period. Estrogen and progesterone levels decline, and the lining of the uterus sloughs off. Bleeding can last anywhere from 2-8 days, with some women reporting shorter or lighter periods, and others longer or heavier ones. As the uterine lining is shed, hormones start changing again, with estrogen levels rising, causing the lining of the uterus to thicken and an egg in the ovary to mature. How quickly this happens after a period starts depends on each individual person.
Once the egg is ready to be released, there is a surge of hormones that cause the egg to leave the ovary and enter the fallopian tube. This is called ovulation, and it is accompanied by a rise in the hormone progesterone. At this point, the egg is ready to be fertilized. If it is fertilized, it travels down the tube and implants in the uterus about a week after conception. The high levels of progesterone make sure the uterine lining is ready for this implantation. But if fertilization does not occur, the egg disintegrates and is reabsorbed into the body. About two weeks after the egg is released, the cycle ends, and a new cycle begins with the starting of another menstrual period.
The phases of the menstrual cycle are standard to all women of childbearing age, though exceptions do occur, such as with PCOS (polycystic ovarian syndrome), and changes that occur naturally as a woman is nearing menopause. Additionally, there are typically about 14 days between when ovulation occurs and the start of her next period, with usually only a few days variation between women. What varies most often is the length of time during the first half of the menstrual cycle.
For example, if a woman’s cycle is only 21 days long, she most likely ovulated around day 7 of her cycle. On the other hand, if a woman’s cycle was 35 days, she probably ovulated closer to day 20.
The difference in when a woman ovulates makes a difference in when she is considered fertile. The “fertile window” is the days during a menstrual cycle that a woman could potentially get pregnant if she is sexually active. The fertile window is often considered to be 5 days before through 48 hours after ovulation. There are two factors that are considered when identifying the “fertile window”. First, sperm can stay alive in a woman’s body for up to 5 days, so even if a woman doesn’t have sex on her ovulation day, if she did within the previous 5 days, she could still conceive. Additionally, the egg can stay alive for 24-48 hours after ovulation, so she is still able to conceive during the day or two after she ovulates.