FAM Basics: The uterus

The uterus doesn’t just matter when you’re pregnant (or trying)
uterus function, how big is the uterus, where is the uterus located, anatomy of the uterus

Ah, the uterus, the womb. Everyone’s first home and the best dark, cozy hot tub any developing baby could want. This amazing organ prepares each month to receive a newly created blastocyst, houses him or her through embryonic and fetal development, and then expels him or her out into the world. During that process, the uterus grows from just three inches tall and an inch-and-a-half wide to the size of a watermelon… and then shrinks back after the baby is born! In this installment of our FAM Basics series, we’ll explore uterine anatomy plus the functions of the uterus both during and outside of pregnancy. 

Uterus anatomy

The uterus is a small, hollow organ composed of three layers. The perimetrium is the protective outermost layer. The myometrium is the middle layer of strong, smooth muscle. The innermost layer, the endometrium, is itself made up of two layers. The endometrium’s basal layer is stable and secures its ever-changing second layer, the functional layer, to the myometrium. Estrogen signals that functional second layer, composed of mucus and connective tissue and with lots of tiny blood vessels, to thicken in preparation for receiving a fertilized egg. 

Uterine shedding, aka menstruation

In the absence of a fertilized egg (which is most accurately referred to as a blastocyst, since fertilization produces a new human organism genetically separate from the egg), the functional layer will be shed and expelled through the vagina with the help of uterine contractions after the corpus luteum breaks down. Interestingly, all placental mammals have a uterus. But humans are one of the few species to menstruate. Most other mammals reabsorb their endometrium at the end of each hormone cycle.

What the uterus looks like

The uterus itself is shaped like an upside-down pear, wider at the top and narrower at the bottom. The topmost section is called the fundus and connects to the fallopian tubes. The middle section of the uterus is called the corpus or main body of the uterus. The lower section is called the lower neck, which is capped by the cervix, separating the uterus from the vagina. 

Where is the uterus located? 

Anatomically, the uterus rests between the rectum and bladder and can lie in a variety of positions. It could be anteverted, anteflexed, retroverted, retroflexed, midline, or angled left or right [1]. “Ante-” means the uterus tilts towards the bladder and “retro-” means it tilts towards the rectum. “-Verted” means tilted, and “-flexed” refers to a uterus that looks bent in that direction. Positioned towards the bladder (anteverted or anteflexed) is the more common position. In rare cases, a retroverted uterus can cause a dangerous condition in pregnancy called an incarcerated uterus [1].

How big is the uterus?

If you place your thumbs together side-by-side, you’ll have a good estimate of the uterus’ normal size. It’s about 8cm x 4cm x 5cm or 3.14 inches x 1.57 in x 1.97 in [1]. During pregnancy, your OB or midwife will usually take a measurement called fundal height. This measures from the bottom of your uterus to the top (fundus) of the uterus. By happy coincidence, this measurement in centimeters will typically correspond to how many weeks pregnant a woman is. So, at 30 weeks, the fundal measurement should be 30 cm, or one foot. 

By the end of pregnancy, the uterus is about the size of a watermelon and weighs a whopping two pounds all on its own. Following birth, the uterus gradually shrinks, called involution, over the next six weeks. According to Cleveland Clinic, the top of the uterus will come down by one to two centimeters or 0.4 inches to 0.7 inches each day following birth. And it will lose half of its mass in the first week postpartum alone!

Functions of the uterus

While not pregnant, the uterus either prepares for pregnancy or for the cyclical reset we know as menstruation. In pregnancy prep mode, the endometrium is thickened for possible implantation and undergoes small uterine contractions (yep, your uterus doesn’t just contract during labor!). During ovulation, estrogen signals to the uterus to contract in a way that favors sperm transport, helping sperm make their long journey to the fallopian tubes [2]. Orgasm may also initiate these upward contractions that assist sperm. 

How the uterus prepares for menstruation (your period)

During the luteal phase, following ovulation, the uterus has opposing contractions that result in no net movement, in order to create a stable environment for an egg to implant. During menstruation, the uterus contracts to empty itself of the sloughed endometrium. Contractions during this time range from “achy” to intense. Also, since the uterus and the digestive system are both composed of smooth muscle, many women experience gastrointestinal issues during menstruation. Progesterone signals smooth muscle to get rid of whatever is inside it, so… you may lose both period blood from the uterus and “period poop” or loose stool or diarrhea from the intestines. If this happens to you, be sure to drink lots of water to replenish lost fluids and eat foods high in fiber.

The uterus in pregnancy mode

Doing all the behind-the-scenes work of set-up and take-down during the various stages of the menstrual cycle is just the uterus’ day job. It’s what it does to pay the bills while waiting for a chance to fulfill its dream and do what it was made for: support a developing fetus during pregnancy and then help its little occupant enter the great wide world. As mentioned previously, the first step is to create a nice, thick, functional endometrium for the egg to implant. Following implantation, the endometrium develops further and supplies the embryo with nutrients while the placenta develops to take over the role of sustaining the developing embryo. For the rest of pregnancy, the uterus’ job is to expand to accommodate the growing fetus. Then, showtime! 

The uterus during labor

In labor, the uterus responds to oxytocin in a positive feedback loop. Most of the biochemical processes in the body utilize a negative feedback loop, in order to self-regulate whatever reaction is occurring, such as producing a specific enzyme. Birth, however, is meant to be a no-going-back kind of process. 

As the fully-developed baby stretches against the walls of the uterus and the cervix, he or she stimulates the mom’s pituitary gland to release oxytocin, which increases uterine contractions, which places more pressure on the uterus and cervix, which stimulates a release of oxytocin… so on and so forth [3]. 

The uterus during birth

Besides its role in the hormonal process of birth, the uterus itself, as a “bag of muscles” has an important role in the physical process of birth. The longitudinal (up and down) muscles of the uterus contract to pull back and open the cervix, while the circular muscles conversely need to relax and lengthen [4]. This process continues until the cervix is fully dilated. In pushing contractions, the top of the uterus continues to push down on the baby, and the mother usually feels an urge to bear down along with these contractions.

Then, after lots of work and pushing, or sometimes in no time at all, the baby is born! The placenta then detaches and delivers. Next, the uterus begins the involution process to shrink back to its original size, assisted by the oxytocin released during breastfeeding. (What about moms who don’t breastfeed? Their uteruses shrink down too, just less rapidly!) In other words, when it matters most, the uterus sure is a hard-working muscle! 

The bottom line

Ultimately, the uterus can be thought of as “the main stage” for a woman’s menstrual cycle, and the reason why that cycle exists: to conceive, grow, nurture, and deliver a brand-new little person into the world. Charting your cycle can help you understand when these changes occur and take the mystery out of your monthly cycle… and maybe improve your relationship with this special, muscular organ that works hard to make pregnancy possible. 

References:

[1] Ameer MA, Fagan SE, Sosa-Stanley JN, et al. Anatomy, Abdomen and Pelvis: Uterus. [Updated 2022 Dec 6]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470297/

[2] Bulletti, C et al. “Uterine contractility during the menstrual cycle.” Human reproduction (Oxford, England) vol. 15 Suppl 1 (2000): 81-9. doi:10.1093/humrep/15.suppl_1.81

[3] Sadava, D., and Purves, W., and Orians, G., and Hillis, D., and Heller, H.C. (2009) Life: The Science of Biology, 9th edition. W.H. Freeman. 

[4] McCutcheon, S. (2018). Natural Childbirth the Bradley Way. Plume. 

Additional Reading:

The surprising states of the uterus explained

FAM Basics: Everything you need to know about your endometrium

“The Love Hormone:” What does oxytocin do for us? 

FAM Basics: Caring for your cervix

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