A quick science blurb on pregnancy
>Let’s dive into the physiology of pregnancy. This will help create a better foundation as you read through our blog posts.
>Pregnancy is the state in which the female body prepares for and nurtures a fetus until delivery. After the process of fertilization, the event in which an egg is fertilized by a sperm, the female body begins to undergo multiple complex physiologic changes to adapt for the growth of a baby.
>Let’s first look at the anatomy of the female reproductive system before we dive in.
- Vagina → Muscular canal from outside the body to the uterus, also called the birth canal
- Cervix → Lower part of the uterus that connects the vagina to the rest of the uterus; protects the uterus from the outside elements
- Endometrium → Inner, mucous membrane layer of the uterus; part of the uterus that responds to progesterone, estrogen to prepare for implantation; provides nutrients to the embryo until the placenta forms; part of the endometrium is shed during menstruation.12
- Fallopian tube → Passageway from the ovary to the uterus
- Ovary → Produces, stores, and release eggs, as well as hormones, like estrogen
>As female sex hormone (progesterone, estrogen) levels fluctuate, beginning after conception, a series of signals are sent throughout that body that contribute to a multi-organ shift.
Hormones and uterine/ovarian changes:
- There are two cycles that are dependent on one another to create the environment needed for a baby to grow.
- The uterine cycle:
- Menstruation → proliferative phase (thickening of the uterus) → ovulation → secretory phase (maintaining uterine thickness in preparation for a potential pregnancy)12,13
- The ovarian cycle:
- Follicular phase (maturation of the follicle in preparation for release of the egg out of the ovary)→ ovulation → luteal phase (creation of the corpus luteum which secretes progesterone to maintain the uterine lining (endometrium) for pregnancy)12,13
- The uterine cycle:
- Hormones:
- Estrogen → leads to thickening of the endometrium (lining of the uterus), and another hormone, luteinizing hormone (LH) , to surge leading to ovulation and increase in amounts of cervical mucus to help promote sperm transit.12, 13
- Progesterone → maintains the endometrium after ovulation13
If you want to learn more about the complex hormonal pathways, and other processes involving the reproductive system, I highly recommend reviewing reference #13: https://openstax.org/books/anatomy-and-physiology-2e/pages/27-2-anatomy-and-physiology-of-the-ovarian-reproductive-system
>Let’s put all of the pieces together:
>What are some of these bodily changes that occur during pregnancy?
| Systems | Bodily changes |
| Cardiovascular | – Increase in cardiac output (total volume pumped by the heart each minute to meet the body’s physical demands), and plasma volume (component of the blood other than red blood cells and platelets)1,2 – Decrease in blood pressure until closer to term1,4 – Lowest drop in pressures around 24 weeks11 – Because of these changes, swelling in the legs, heart murmurs, rhythm changes detectable on an ECG11 |
| Hematologic (related to the blood, lymphatic system) | – An increase in plasma volume, as stated above, creates an anemia of pregnancy, due to the blood being more diluted.4,5 – Increased clotting factors; a decrease in proteins that inhibit clotting – Create a state of hypercoagulability (increased state of clotting) that decreases the risk hemorrhage in delivery |
| Respiratory | – Diaphragm elevates11 – Increase in respiratory rate (amount of breaths and volume intake with each breath) – Increase in oxygen consumption, especially by the 3rd trimester |
| Renal (kidneys, bladder) | – Increase in hormones secreted by the adrenal glands11 – Increased blood flow through the kidneys leads to increased filtration rate by the kidneys7 |
| Endocrine (hormone response) | – Increase in progesterone, estrogen levels stimulate processes to occur all over the body as highlighted here in this graph.Increase in oxytocin (“the love hormone”)11 – Increase in prolactin → stimulates breast tissue and duct production for lactation11 – Insulin resistance Insulin → hormone that promotes glucose (sugar) to be stored in our organs and used for energy. Resistance, meaning that this temporary state = less sugars stored into our organs so that glucose can be transferred to the growing fetus Increase in hCG (produced by the placenta) to support fetal growth8,9 – Elevated melanocyte (pigment cells in our skin)-producing cells leads to hyperpigmentation (darkening of the skin). Areas include the areolas, face, abdominal line, and umbilicus (belly button)11 – Increased human placental lactogen (hPL) is produced by the placenta and leads to major changes in nutrition diverted from mom to baby → increase in fats, sugars to mom and baby – Fats are essential for steroid production in the fetus (which are made out of fats) – Vitamins and minerals |
| Gastrointestinal | – Increased estrogen, progesterone, and hCG (stimulated by the placenta) can cause nausea and vomiting (aka morning sickness). – These symptoms can also be produced by decreased blood sugar (hypoglycemia)11 – Decrease in gut motility, which can increase constipation, reflux10 |
| Reproductive | – Progesterone stimulates the uterine wall to thicken; relaxes the muscles in our uterus to prevent contraction during fetal growth11 – Growth of the uterus throughout pregnancy is stimulated by increased estrogen11 |
| Musculoskeletal | – Shift in center of gravity that increases bend in in the spine and neck, which can contribute to lower back pain11 – Joint laxityWidening of sacroiliac joint → joint between spine and the pelvis, prepares for delivery11 |
>What is the typical timeline of pregnancy?
- From implantation to birth, the duration is about 266 days
- Pregnancy from last day of menstrual period (LMP) to birth is ~280 days
- Delivery timing
- Early term: 37 0/7 weeks – 38 6/7 weeks
- Full term: 39 0/7 weeks – 40 6/7 weeks
- Late term: 41 0/7 weeks – 41 6/7 weeks
- Post term: 42+ weeks
>References
- Journal of the American College of Cardiology. 2019. Halpern DG, Weinberg CR, Pinnelas R, et al.
- Journal of the American College of Cardiology. 2016. Elkayam U, Goland S, Pieper PG, Silverside CK.
- Hypertension. 2023. Alexander BT, South AM, August P, et al.
- Obstetrics and Gynecology. 2019. Committee on Practice Bulletins—Obstetrics
- The Journal of the American Medical Association. 2005. Kaaja RJ, Greer IA.
- Respiratory Medicine. 2025. Ejikeme C, Nandakumar V, Gotur D.
- Clinical Journal of the American Society of Nephrology : CJASN. 2012. Odutayo A, Hladunewich M.
- Journal of Diabetes Research. 2019. Kampmann U, Knorr S, Fuglsang J, Ovesen P.
- Diabetes/Metabolism Research and Reviews. 2003. Di Cianni G, Miccoli R, Volpe L, Lencioni C, Del Prato S.
- The American Journal of Gastroenterology. 2022. Elkins JR, Oxentenko AS, Nguyen LAB.
- Pascual ZN, Langaker MD. Physiology, Pregnancy. [Updated 2023 May 16]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 Jan-.
- Dias Da Silva I, Wuidar V, Zielonka M, Pequeux C. Unraveling the Dynamics of Estrogen and Progesterone Signaling in the Endometrium: An Overview. Cells. 2024; 13(15):1236. https://doi.org/10.3390/cells13151236
- Betts JG, Desaix P, Johnson E, et al. Anatomy and Physiology, 2E. OpenStax, Rice University; 2022.
- Huffman, J.W. (2026, May 9). pregnancy. Encyclopedia Britannica. https://www.britannica.com/science/pregnancy
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