8.1: The Beginnings of Development
- Page ID
- 228234
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- Explain the process of meiosis and its role in the creation of gametes.
- Describe the menstrual cycle and the hormonal changes that trigger ovulation.
- Define the “fertile window” and explain its significance in relation to conception.
- Evaluate common questions about conception risk and fertility patterns based on current evidence.
The Beginning
A biological female is born with all the ovum (eggs) she will have- between 1 and 2 million. By the time the female enters puberty, only about 25%, or 300,000, remain. This number steadily declines as she ages. On the contrary, once a biological male reaches maturity, his body is capable of producing sperm. The process responsible for creating ovum and sperm (collectively known as gametes, or sex cells) is meiosis. Meiosis is a cellular division process that produces gametes that have 23 chromosomes instead of 46 chromosomes. By reducing the number in half, it ensures that when fertilization of the egg by sperm occurs, the offspring receives the correct number of chromosomes.
Ovulation and Becoming Pregnant
The prenatal stage begins before a woman is even aware of being pregnant. The prenatal journey begins on the first day of the last menstrual period (LMP). This date is what will be used to determine the baby's estimated due date (EDD). An early ultrasound in the first trimester may be used to help confirm EDD. About two weeks before the anticipated next menses, a follicle in a woman's ovary matures and releases an egg into the fallopian tube, where it awaits sperm. The uterine lining also thickens in preparation for the implantation of a fertilized egg. This is called ovulation, and only happens once a cycle. While ovulation can be somewhat predicted, it's difficult to say exactly when it will occur. What we do know is that once ovulation has occurred, menses follow 14 days later.
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Figure \(\PageIndex{1}\). Menstrual Cycle Chart. Image by Gaboka86 is licensed CC-BY-SA 4.0
In the 24-36 hours preceding ovulation, a chemical called luteinizing hormone (LH) is released, triggering ovulation and supporting the early stages of pregnancy (should it occur). Once the egg is released, it can survive for 12 to 24 hours. Sperm, on the other hand, can live for up to five days in a biological female's reproductive system.
Therefore, intercourse occurring between five days before and 24 hours after ovulation can result in pregnancy. This is referred to as the fertile window. For the example below, the fertile window would be Days 8-15. Ovulation typically occurs between days 13 and 14, so the fertile window begins on day 8 and lasts until the day after ovulation (day 15).
Figure \(\PageIndex{2}\). Risk of Conception. Image by Genetics4good is licensed CC-BY-SA 3.0
Frequently Asked Questions
- Can a female get pregnant during her menses? The short answer is yes, but it is highly unlikely. Ovulation can overlap with menses if she ovulates earlier in the cycle and experiences longer menstrual cycle symptoms.
- What are the chances of getting pregnant each cycle? The literature varies, but most agree that there is approximately a 30-35% chance of conceiving each cycle when no intervention is used.
- What happens if a female doesn't use any form of birth control, including the calendar method or condoms? There is an 85% chance of becoming pregnant within a year.
- Do the chances of getting pregnant vary based on the day of intercourse and the timing of ovulation? Yes. The timing of intercourse, based on when ovulation occurs, does impact the likelihood of conceiving.
Table \(\PageIndex{1}\). Chance of conception by percentage based on the timing of intercourse.
| Day | Chance of Conception (%) |
|---|
| 5 days before ovulation | ~5-10% |
| 4 days before ovulation | ~16% |
| 3 days before ovulation | ~25% |
| 2 days before ovulation | ~27% |
| 1 day before ovulation | ~31% |
| Day of ovulation | ~35% |
| Day after ovulation | ~8% |