Maternity Notes
Physiological adaptations to pregnancy
Cardiovascular
30 - 50% increase in cardiac output.
Peaks in trimester 3
Due to a rise in estrogen levels
Increase in preload to the heart which has an impact on the hearts ability to stretch.
Increase in HR, which will not exceed 100 bpm. There is a drop in BP, mediated by progesterone as it decreases systematic vascular resistance.
10-15mmhg in the first trimester
Normal in the second.
Uterine growth can effect peripheral vascular return.
Respiratory
Renal and Urinary
Gastrointestinal
Haematological
The fetal membranes or amniochorionic membranes.
The amniochorionic membranes are essential for the protection of the fetus, maintenance of pregnancy, and sometimes as a signal to initiate labour. These membranes start their growth and development at the time of implantation: the inner membrane, or amnion, derives from the inner cell mass of the blastocyst and it is bathed in amniotic fluid and in constant touch with the fetus. The outer membrane, or chorion, derives from the trophoblasts and it is continuous from the base of the placenta.
Amniochorion provides mechanical, immune, endocrine, transport, and antimicrobial functions during pregnancy (Menon, 2021).
The placenta at term.
The mature placenta is an oval/round disc, about 20cm across and 2-3cm thick. The margins of the placenta are continuous with the fetal membranes (see picture below, left). On average, the placenta weights about 1/6 of the weight of the fetus. The amniotic membrane is smooth so the fetal aspect of the placenta (see picture below, middle) appears shiny and grey. The maternal side of the placenta (see picture below, right) is dull and is subdivided into lobes.
The umbilical cord is normally between 50-60cm long. The vessels of the cord (2 arteries and 1 vein) are embedded in Wharton's jelly, which is connective tissue that protects these vessels.
Amniotic Fluid.
Amniotic fluid is produced by the amniotic membrane and has an important role in protecting the fetus, cushioning from external impact. It also allows symmetrical fetal growth and movement, and maintains a constant body temperature. The composition of the amniotic fluid varies as pregnancy progresses; during the first trimester amniotic fluid consists mostly of water and electrolytes and contains minimal protein. However, by 25 weeks of gestation keratinization of the fetal skin, resulting in a decrease in the permeability of fetal tissues to water and solutes. This process, along with the with the ability of the fetal kidneys to produce urine, results in increased amniotic fluid concentrations of urea and creatinine, decreased concentrations of sodium and chloride, and reduced osmolality. By the end of the third trimester, a variety of carbohydrates, proteins, lipids, electrolytes, enzymes, and hormones, which vary in concentration depending on the gestational age, are also present.
By term, the normal volume of amniotic fluid is 500-1000ml. Polyhydramnios is an excess amount of fluid (over 2000ml), normally associated with multiple pregnancies or fetal swallowing problems. A deficiency in amniotic fluid is know as oligohydramnios (less than 500ml), often associated with fetal renal problems.
Fetal Development
Embryological development is complex and occurs from week 2-8. Fetal development is the period from week 8 gestation until birth.
Image from Myles, 2014
First trimester. Frist trimester is considered a period or organogenesis, which is the most critical period in fetal growth and development.
Second trimester. During the second trimester growth and development continues.
Third trimester. This is the period of most rapid growth, and is mainly focused on weight gaining.
Fetal Circulation
As the fetal oxygen source is the placenta rather than the lungs, blood in the fetal circulation flows in an adapted circuit that perfuses the placenta and bypasses the lungs. in order to do this, fetal circulation has a number of adaptations:
1. Umbilical Vein is a vein present during fetal development that carries oxygenated blood from the placenta into the growing fetus
2. Ductus venosus is a shunt that connects the intra-hepatic portion of the umbilical vein to the inferior vena cava
3. Foramen ovale allows blood to move from the right atrium to the left atrium.
4. Ductus arteriosus connects the pulmonary arterial trunk to the descending aorta.
5. Umbilical artery is a paired vessel that stems from the anterior division of the internal iliac artery returning to the placenta.
Fetal Skull
For midwives and obstetricians it is important to understand the parameters and characteristics of the fetal skull because of its significance during the mechanisms of labour. By assessing landmarks of the fetal skull, midwives are able to diagnose the position of the fetal head in the pelvis and determine the most likely mechanism of labour and mode of delivery.
Fetal skull has 2 main functions: 1) protection of the brain; 2) ability to change shape, adapting to the process of labour .
The skull is divided into 3 parts: the vault, which is a large, dome-shape compressible area, the base comprises bones that protect the vital centres in the medulla, and the face. Bones of the base and the face are unable to compress and therefore do not have the ability to mould during labour.
Picture from Annamma, 2015
Bones of the vault.
The bones of the vault are not fused at birth, leaving small gaps between the bones, known as sutures and fontanelles. Ossification of the skull is not complete until early adulthood. Below you can see a diagram with the different bones of the vault (Left diagram from Darbas and Lohrenge 2018; Right diagram from Annamma, 2015)
The occipital bone lies the back of the head. There are two parietal bones lying on either side of the skull. These are the two largest bones in the skull and are united at the top of the skull by the sagittal suture. There are two frontal bones form the forehead, also known as sinciput. There are also two temporal bones at each side of the head.
The sutures are the cranial joints where two bones meet. Where two or more sutures meet, a fontanelle is formed. These structures allow a degree of overlapping of the skull during labour.
Regions and landmarks of the fetal skull
The skull is further separated into regions and within these are important landmarks. During labour, during a vaginal examination, midwives feel for these landmarks to determine the position of the fetal head.
Occiput is the area over the occipital bone.
Vertex is the area bordered by the anterior fontanelle, the posterior fontanelle and the two parietal eminences.
Sinciput or brow (forehead) is the area covering the frontal bones
Face it extends from the orbital ridges to the chin or mentum. The point between eyebrows is called the glabella. Face cannot mould during labour. If the face is the presenting part in labour the face may sustain severe swelling and bruising (see below).
Bruised face baby after face presentation delivery
Moulding of the fetal skull.
The fetal skull has the ability to adapt to prolonged compression to aid the passage through the birth canal. this adaptation is known as moulding. Moulding can increase and reduce the skull diameters to a certain degree, enabling the skull to change shape, but not volume. This is also a protective mechanism to prevent the fetal brain being compressed. Posterior moulding reduces within hours of birth, whereas anterior moulding decreases over the first 48 hours of birth.
Implication for Paramedic Practice.
Even though as Paramedics you do not perform vaginal examinations, you do not need to learn the exact measurements of these diameters and identify different landmarks, however this content will help you understand that there are a lot of different factors that influence the progress of labour. Also, understanding the process of moulding could help you reassure parents that the 'odd head shape' of a baby is a normal adaptation to labour and that it will be resolved within the first 2 days of birth.