What are the major parts of the female reproductive system?

There are several key parts that make up the female reproductive system:

• ‍Ovaries: These two structures create the eggs (oocytes), which develop inside fluid-filled sacs called follicles. The ovaries releases eggs and have a hormonal function as well: making and secreting the hormones estrogen and progesterone, as well as smaller amounts of testosterone.^i,ii
• ^‍Uterus: The embryo implants and develops in this hollow organ. It has two parts —the corpus (body) and cervix (bottom part of the uterus that is the path from the corpus to the vagina).ⁱⁱⁱ A female’s uterus has three layers:
  • ‍Endometrium: The inner uterine lining or cavity that is shed during menstruation. Its thickness varies by the stage of the menstrual cycle in response to changes in hormone levels.^iv
  • ^‍Myometrium: This thickest uterine layer, made up of smooth muscle, is the middle layer. It contracts to help sperm pass through the uterus to reach the egg. It also helps the embryo move into and along the uterus until implantation.^v,vi
  • ^‍Serosa: The outermost layer is a protective covering that prevents the uterus from sticking to neighboring organs.^vii
• ^‍Cervix: Although it is part of the uterus, the cervix is often thought of as its own entity. This tubular part connects the bottom of the uterus to the vagina, opening into the upper vaginal area. During natural conception, sperm passes from the vagina through the cervix into the uterus.^viii
• ^‍Fallopian tubes: These two tubes serve as passageways between the uterus and each ovary. When eggs develop inside follicles and are released from the ovaries, they travel through the fallopian tubes and are fertilized there by the sperm. The fertilized egg then moves to the uterus and implants in the uterine wall.^ix
• ^‍Vagina: This muscular, tubular structure connects the vulva to the cervix.^x
• ^‍Vulva: This outer part of the female genitalia consists of the labia majora, labia minora, and clitoris.^xi

What is the difference between a follicle and an oocyte?

Oocytes develop inside cystic structures known as follicles. The earliest stage of follicles, known as primordial follicles, develop at the beginning of each menstrual cycle. One becomes dominant, and the others do not develop further. The dominant follicle continues to develop and the oocyte inside it resumes meiosis and develops into a mature oocyte, also known as an egg. During ovulation, the dominant follicle ruptures, releasing the egg to be caught by the fallopian tube.^xii

In what phase of the menstrual cycle does conception occur?

Typically, a menstrual cycle ranges from 25 to 30 days. It includes both the ovarian cycle (where follicles develop in the ovaries) and the uterine cycle (which involves the thickening of the uterine lining (endometrium)).^xiii

The ovarian cycle has two phases: follicular and luteal. The follicular phase can vary in length, while the luteal phase is more consistent and typically lasts around 14 days.^xiv

• ‍Follicular phase: This phase starts with the onset of menstrual period bleeding, when the endometrial lining is shed. The phase lasts throughout menstruation and ends at ovulation. Ovulation occurs 34 to 36 hours after the start of the luteinizing hormone (LH) surge and 10 to 12 hours after the peak of the LH surge.^xv
• ^‍Luteal phase: This phase starts after ovulation and ends when menstruation occurs or when conception takes place. If fertilization occurs, it happens during the luteal phase, after ovulation.^xvi

After ovulation, the mature egg (or oocyte) enters the fallopian tube. The egg will stay inside the fallopian tube and can survive for about 24 hours. During this window, conception or fertilization most often occurs. In natural conception, sperm travel up the cervix, into the uterus, and up into the fallopian tubes. A sperm may meet one of the ovulated eggs in the fallopian tube and fertilization may occur.^xvii

The fertilized egg is now an early-stage embryo (zygote). It slowly passes through the fallopian tube for three to four days, eventually reaching the uterus. The embryo (now a group of cells called a blastocyst or Day 5 or 6 embryo) drops into the uterus where it must implant into the lining of the uterus to continue developing.^xviii

When is the fertile window?

This window is the time during which fertilization (conception) can occur.^xix

What is occurring anatomically during the fertile window?

The egg must be exposed to a sperm during this time for fertilization to occur.^xx Sperm can survive in a female’s reproductive tract for up to five days,^xxi,xxii but the average lifespan is 1.4 days.^xxiii Because sperm have a potential five-day survival and the egg stays in the fallopian tube for up to 24 hours, the fertile window can span from five days before the ovulation day.^xxiv

Ultimately, there is an approximately six-day window for an egg to be fertilized and for getting pregnant. On average, the fertile window is from day 8 to day 14 of the menstrual cycle.^xxv

What are hormones doing during the fertile window?  

Roughly three days before ovulation, estradiol rises to peak levels, indicating the start of the fertile window.^xxvi This rise eventually triggers the luteinizing hormone (LH) surge where a large amount of LH released from the pituitary gland), which takes approximately 24 hours. At the same time, a smaller follicle-stimulating hormone (FSH) surge occurs.^xxvii Ovulation occurs 34 to 36 hours after the LH surge starts and sharply declines once the surge is over.^xxviii

When the LH surge starts, estradiol slowly declines until after ovulation, after which it begins to increase again, until immediately before menstruation, when it declines again.^xxix

Progesterone, which is low during the fertile window, also rises during the LH surge and peaks about seven days after ovulation.^xxx

How does an ovulation predictor kit (OPK) work?

Ovulation predictor kits (OPKs) are urine tests that are used to indicate the fertile window. Some measure LH only, while others measure both LH and estrogen metabolites (estrone-3-glucuronide or E3G) found in urine.^{xxxi,xxxii,xxxiii,xxxiv,xxxv,xxxvi,xxxvii}

These kits cannot definitively determine when ovulation will occur, but they give individuals pursuing conception a good idea of ovulation timing. Typically, ovulation occurs roughly 36 hours (between 24 to 48 hours) after the LH surge starts.^xxxviii

It is worth noting that some females will have an LH surge but will not release an egg. In these cases, OPKs will indicate the surge, but ovulation will not follow.^xxxix There is also a chance for a false positive result on a home OPK. Therefore, some providers may prefer to confirm a positive OPK with an office blood test or ultrasound if OPKs are being used to time intrauterine insemination (IUI) or as an aid for females with infertility.

OPKs that measure both LH and E3G may help pinpoint the window's start, potentially indicated by an estrogen rise. This elevation typically occurs three days before the LH surge. Consequently, these OPKs may identify the window a few days before LH-only kits.^xl

What happens if you have sex during the fertile window?  

During the window, with unassisted conception, semen (containing sperm) is ejaculated into the vagina. Within five to 10 minutes, the sperm can travel through the cervix into the uterus and fallopian tubes.^xli,xlii Once there, if a sperm meets an egg, fertilization may occur.

What is the role of sperm?

If semen parameters are optimal, each ejaculation contains 1.5 to 5 mL of semen — and between 200 to 500 million sperm.^xliii After intercourse, the semen pools in the vagina near the opening of the cervix, called the cervical OS.^xliv

While only one sperm is needed for fertilization, having a high number is critical because not all sperm will be viable, and they will face several obstacles on their journey to the egg. First, sperm must overcome the vagina’s naturally low pH. While sperm's ideal environment is between 7.0 and 8.5 pH (more basic or alkaline), the normal vagina is more acidic, ranging between a pH of 3.5 to 4.^xlv,xlvi

Fortunately, both seminal fluid and cervical mucus are more alkaline, counteracting the vagina’s pH and allowing sperm to survive.^xlvii One study from the 1970s showed that the interaction between the seminal fluid and the vagina raises the vagina’s pH to 7 after ejaculation, creating a good environment for sperm survival.^xlviii

Immediately after ejaculation, the seminal fluid coagulates and becomes jelly-like. This step keeps the sperm inside the vagina near the cervix and protects the sperm from an acidic vaginal environment. Sperm begin to exit the coagulated semen immediately and pass through the cervix, with some sperm reaching the fallopian tubes within five to 10 minutes.

Then, substances within the semen gradually liquify the coagulated semen in the vagina. Within 30 minutes, sperm remaining in the vagina have reduced motility because of exposure to the acidic vaginal environment. After about one hour, the remaining semen is entirely liquid and typically flows out of the vagina with the remaining non-motile sperm.^xlix,l

Once the sperm reach and enter the cervix, the pH is higher and therefore more hospitable. During ovulation, the pH of the cervix is even better for sperm survival, facilitating sperm entry and fertilization.^li After sperm travel past the cervix, it is thought that uterine contractions assist the sperm in passing through the uterus to the fallopian tubes.^lii Fluid in the tubes, which increases during ovulation, helps propel the sperm forward to help them get to the egg.^liii

There are some barriers along the way. For example, some sperm, particularly those with abnormal structure, may struggle to pass through the cervical mucous or fallopian tubes.^liv,lv Ultimately, millions of sperm enter the uterus, but studies show only a few hundred make it to the fallopian tubes.^lvi

Fertilization and the early embryo

A process called capacitation is required before ejaculated sperm can fertilize an egg. Capacitation is a series of changes to the structure of the sperm as it moves from the cervix to the egg’s location in the fallopian tube; these changes include the sperm exposing special receptors that guide it to the egg’s outer layer. Capacitation also stops sperm with poor motility or morphology from reaching the egg. It is believed that features of the female reproductive tract (such as exposure to cervical mucus) trigger capacitation.^{lvii,lviii,lix,lx,lxi,lxii}

The acrosome reaction is the last step for sperm before they are ready for fertilization. Once the sperm reaches the egg’s outer shell, known as the zona pellucida, the cap over the sperm’s head (acrosome) releases chemicals. This reaction helps the sperm break through the zona pellucida.^lxiii,lxiv It takes roughly five to 20 minutes for the sperm to break through and fuse to the plasma membrane inside of the egg. Once complete, the zygote (the earliest embryo stage) forms, and fertilization has occurred.^lxv

The zygote now thickens and strengthens the zona pellucida, preventing penetration by any other sperm.^lxvi Within one to three minutes of fusing with the plasma membrane, the sperm loses motility .^lxvii

As the zygote moves down the fallopian tubes toward the uterus, it divides along the way. The single-celled zygote divides into two cells and then four cells. By Day 3, it is eight cells. On Day 4, it is a compacted ball of cells called a morula, and by this point it has typically reached the end of the fallopian tube and drops into the uterus. Once in the uterus, the embryo continues to divide, becoming a blastocyst on Day 5 or 6.^lxviii At this point, the blastocyst hatches out of the protective zona pellucida.^lxix

What is implantation?

The process called implantation occurs when the embryo attaches to the endometrium where it stays throughout pregnancy. The upper and back walls of the uterus are common implantation locations.^lxx

For implantation to occur, the endometrium must be receptive and have adequate thickness. Thickness varies based on the day of the menstrual cycle and from person to person. Typically, the endometrium thickens to a peak level around ovulation to prepare for implantation. Ultimately, before the embryo implants, it must reach the uterus when the endometrium is suitable for implantation with adequate nutrients.^lxxi,lxxii

Two parts of the blastocyst will continue to develop — the inner cell mass (which will develop into the embryo) and the trophoblast (which develops into the placenta). The trophoblast attaches to the receptive endometrium.^{lxxiii,lxxiv,lxxv}

Implantation is an essential part of pregnancy. The uterine lining provides the initial nutrients required by the growing fetus until the placenta takes over. Through implantation, the growing embryo gets enough sustenance to continue developing.^lxxvi

Some females do have symptoms with implantation. The most common are light bleeding, spotting, or cramping pain. These symptoms can sometimes be confused with the onset of menstruation, but they generally last for a shorter time, on average less than three days.^{lxxvii,lxxviii,lxxix} Roughly 25 percent of females have vaginal bleeding during the first trimester. It can coincide with implantation or occur after.^lxxx

In an estimated 1 to 2 percent of cases,^lxxxi,lxxxii fertilization occurs but the zygote implants somewhere other than in the uterus, typically in the fallopian tube. This type of implantation is called an “ectopic pregnancy”.^{lxxxiii,lxxxiv} It is not viable and is typically terminated (using medication or surgery) because allowing the embryo to grow outside the uterus can lead to severe medical complications in the female including death.^lxxxv

What happens if there is no implantation?

If fertilization does not occur or an embryo fails to implant, estrogen and progesterone levels drop steeply around day 28 of the menstrual cycle. Because these hormones support the uterine lining, their decrease results in shedding of the lining (menstrual bleeding). Any embryo that fails to implant will be reabsorbed by the tissues or passed out of the cervix with menstrual blood.

Females typically experience no symptoms if a blastocyst does not implant or stops developing immediately after implantation and is shed with the next period. In many instances, they are unaware there was a fertilized egg and that they were pregnant during these early days. The only sign of a biochemical pregnancy could be a positive pregnancy test that detects the pregnancy hormone (human chorionic gonadotropin, hCG) that is followed a few days later by a negative pregnancy test or the start of a menstrual period.

What role does age play when trying to conceive?

While the sperm plays a significant role in fertilization, the chance of conception for individuals who are trying to conceive (TTC) unassisted varies with the female’s age (i.e., the age of the egg). Table 1 (below) shows conception rates for three age groups. The data comes from a cohort study of 2,962 couples in the U.S. and Canada who were followed for up to 12 cycles. At the start of the study, they had been trying to conceive naturally for three menstrual cycles or less. Although not included in the table, it should be noted that the cumulative pregnancy rate after six cycles for females aged 40 to 45 was 55 percent lower than females aged 28 to 30.^lxxxvi

Conclusion

Having a grasp of the fundamentals of conception can be helpful for couples and individuals seeking a healthy pregnancy. By reviewing this information for educational purposes, patients may find that their physician appointments are more positive because they are better able to follow along and absorb the advice, diagnosis, or treatment plan offered by the fertility specialist.