The uterus, which is in a way our first home, is originally no larger than a pear, with a cavity that does not exceed 4 mL... so how does it manage to house a being weighing several kilos in just a few weeks? The body of the expectant mother changes enormously during the 9 months of pregnancy, and today we are breaking it all down!
The corresponding chart
Anatomical chart of a pregnant woman
This entire article recounts how the uterus displaces its neighbors throughout pregnancy. The chart shows them together in a single image: uterus, fetus and placenta, bladder, intestines, and spine. Hand-drawn by Axel Devillard, printed in France, in two skin tones and two formats.
See the chart30×40 cm at €59 · 40×60 cm at €79 · Free delivery in France
Uterine development during pregnancy
During the first trimester, the uterine fundus gradually rises above the pubic symphysis and thus becomes potentially palpable.
It reaches the navel at approximately 4 and a half months of pregnancy, when the uterus is about 18 cm high. It will then reach 32 cm at term!
But this development does not occur only in height; the gravid uterus gradually widens from 8 to 23 cm by the end of pregnancy.
To give you an idea, the fetus goes from the size of a plum at the end of the 1st trimester to that of an eggplant at the end of the second, and finally to that of a watermelon just before birth! The uterus will then weigh an average of 1 kg and will have a capacity of 4-5L within its cavity.
Uterine ligaments during pregnancy
The main ligaments allowing for the significant uterine distension are the round ligaments and the uterosacral ligaments.
- The round ligaments hypertrophy from the 2nd trimester by multiplying their size by 3 or 4 (in length and width); they then measure more than 10 cm and can withstand up to 40kg of tension!
Because their direction is almost vertical, they can often be perceived by palpation.
- The uterosacral ligaments follow the same thickening process, but the tension of their attachments has a different impact: it makes the rectouterine pouch deeper.
It should also be noted that the uterine artery stretches, uncoils its turns (it is like an accordion waiting for gestation to allow for expansion... clever, right?) and thus also increases its length by 3 to 4 times!
Evolution of uterine anatomical relationships in a pregnant woman
Given its growth, it is obvious that the anatomical relationships of the uterus are modified as pregnancy progresses: in short, it changes neighbors.
At term, it is in contact with:
- Anteriorly with the anterior abdominal wall, which is thinned by the stretching imposed upon it: the linea alba widens, particularly in the umbilical region.
- Posteriorly, the uterus is in contact with the lumbar spine, in particular, as well as the psoas major muscles.
- Superiorly, it is in contact with the transverse colon and brushes against the lower edge of the liver and gallbladder. This forces the uterine tubes, the round ligaments, and the proper ligaments of the ovary to unfold.
- Laterally, the uterus makes room for itself below, but also in front of, the colonic frame. The cecum rises above the iliac crest, for example (whereas it is normally housed within the iliac fossa). In short, all the small bowel loops and the colon tend to move aside and become posterior to make room for the future baby.
The cervix of the gravid uterus
The cervix is the part whose progressive opening will allow the birth of the child.
The dense mucus plug seals it, protecting it and making the uterus "impermeable."
It is essentially during the labor phase of childbirth that it changes (softens) and dilates under the effect of uterine contractions and the hormonal impregnation of the expectant mother.
The formation of the umbilical cord
The umbilical cord is formed from specific embryonic structures between the 1st and 2nd month of pregnancy.
Once newly formed, it continues to lengthen and forms sinuosities in the amniotic cavity. It therefore opts for the same strategy as the uterine artery 😉
Its normal length is 50-60 cm with a diameter of 1.5 cm. It can be too short, though this is rare, or too long, in which case it can wrap around the fetus's neck or shoulders. This represents potential obstetric complications at the time of the child's birth.
This umbilical cord consists of two arteries and one vein, which allow it to ensure fetal-placental circulation. It therefore transmits, among other things, the various nutrients necessary for the growth of the future baby and also relieves it of its waste.
The development of the placenta
During its development, the placenta sends out fine blood projections into the uterine wall, building a structure similar to that of a tree.
The placenta has its origins during embryonic development but is not fully formed until the second trimester, then continues to evolve throughout the pregnancy. It allows for a considerable increase in the exchange surface between the mother and the child, who is increasingly demanding in its metabolic needs. It will weigh up to 500 grams at birth.
The exception of pregnancy denial
A woman's body, unaware that she is pregnant, manages to fully camouflage her pregnancy and all the changes we have just listed.
The belly of these women is completely flat until the realization of the pregnancy, which causes it to round out in just a few hours.
This is made possible by the powerful sheath formed by the rectus abdominis muscles. The uterus is indeed becoming heavier and larger, but it is backed forward against a real muscular wall against which it is impossible to tilt forward. It can then only use the space located above it by pushing the small bowel loops even further upward to develop.
Sources
- Petit bassin et périnée : organes génitaux - Tome 2 (1995), P.Kamina
- Manuel de psychologie clinique de la périnatalité, S.Missonnier (2021)
- Stades du développement du fœtus, Raul Artal-Mittelmark (2021)