Pregnancy: The Phenomenon of Carrying a Developing Fetus

PregnancyThe natural phenomenon of pregnancy; gestation, is the state of carrying a developing fertilised egg in the uterus after conception1. It can be tested for by an over the counter urine test and further confirmed by clinical examinations. Pregnancy is approximately a 9 month or 40-week process from the last menstruation, where a series of complex events follow. These can be split into trimesters which describe the development, from a zygote into a foetus in the uterus; where a foetus is an unborn offspring after the 8-week stage up until birth.

The foetus will have demands which can be resolved by the placenta and umbilical cord2. They have a major role by allowing diffusion to occur; waste is sent back to the mother to be eliminated while nutrients are received from the mother. This highlights how the mother undergoes many changes in order to sustain a successful human pregnancy.Trophoblast invasionSubsequently after fertilisation, the zygote becomes a blastocyst. This is a structure in early pregnancy composed of the inner cell mass that becomes the embryo and an outer layer known as the trophoblast that becomes the placenta.

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The blastocyst will attempt to implant by following the process of pre-attachment, apposition, attachment and invasion. Adhering to the endometrium of the uterus activates development of the placenta. Trophoblast cells differentiate into layers of inner cytotrophoblast and outer syncytiotrophoblast, and the invasion of the decidua and myometrium occurs. This in turn establishes early utero-placental circulation due to the remodelling of maternal vasculature (spiral arteries); which allows foetal blood vessels to be bathed in a pool of maternal blood.

There is a development of foetal vasculature within the chorionic villi which enables the foetus-placental circulation due to the migration of extravillous trophoblast (EVT) from the villi to the placenta. This is followed by the formation of the umbilical cord which is composed of the umbilical artery and vein3 to connect the foetus to the placenta. This establishes the maternal foetal circulation which is a high-flow, low resistance circulation to increase blood flow to the foetus. PlacentaThe placenta plays a key role in pregnancy. It is highly specialised which supports a growing foetus. There are many functions that a foetus is unable to do and therefore, the placenta steps in. It is an exchange surface and acts as a barrier between the blood of the mother and foetus. The placenta mediates the exchange of gas, metabolites, hormones and nutrients via diffusion2 between the mother and foetus without allowing the blood of the foetus and the mother to mix. Additionally, it produces hormones which aid the developing baby to grow and provides some immunity protection by acting as a barrier to bacteria and passing antibodies that the mother already possesses4. Maternal circulation adaptationsThe foetus relies on the mother for its survival which highlights the importance of maternal circulation. Over pregnancy, there are many changes in the maternal circulation of human pregnancy which benefit a developing foetus. For example, there is an increase in blood volume5 during healthy pregnancies. To measure plasma volume in participants, the Evan's blue dye dilution method was used. It concluded that plasma volume begins to increase at approximately 6 weeks of pregnancy. On average, during pregnancy, a woman's blood volume will increase by a total of up to 50%6 ; where after 30 weeks will plateau until labour. The Evan's blue dye may give more accurate results compared to other dyes, but the procedure could affect the results. Results could be more variable when extracting and measuring from the plasma than the serum which invites technical error. However, results were significant enough to indicate blood plasma does expand.The increase in blood volume can be explained by an increase in both plasma volume and red blood cell mass. Despite the fact that the exact mechanism how is unknown, a hypothesis suggests 2 mechanisms that influence it; hormones produced and the maternal circulation acting as an arteriovenous shunt7 . Blood plasma is primarily composed of water, therefore to increase its volume, there will be an increase in water retention and reabsorption. Oestrogen produced by the placenta could stimulate aldosterone which in turn affects the renin-angiotensin system because when Na+ are reabsorbed, water will follow due to the principle of osmosis. Cardiac output (CO); the volume of blood pumped, is calculated by blood pressure (BP)/ total peripheral resistance (TPR). The uteroplacental circulation leads to a fall in vascular resistance which consequently affecting CO. In order to compensate this effect, a cascade of events including influence of atrial natriuretic peptide (ANP) occurs to increase the blood volume, returning it to normal. Red blood cell (RBC) mass increased as a result in increased erythropoiesis8 illustrated by 342 pregnant or post pregnant women who had a blood test. Using flow cytometry, RBC count was totalled indicating erythropoiesis increases the further into pregnancy. Hormones such as placental lactogen9 and prolactin10 have been suggested to drive this mechanism. The benefit of an increased blood volume is to meet the demands of a foetus and allowing it to grow and develop. Increased plasma volume results in an increase in BP and CO which helps deliver nutrients to the placenta and help organs that require a little more oxygen to cope. With a bigger volume, more nutrients can dissolve resulting in higher concentration of nutrients to transport more 11, 12 . RBC production does not increase to the extent of the plasma volume resulting in haemodilution or decreased haematocrit. The benefit of increased RBC mass is that it allows increased oxygen transportation as oxygen carrying capacity has increased. In terms of iron metabolism, there is an increased efficiency of iron absorption from the gut and prejudice towards preserving foetal reserve which aids its development13 . This is further supported by a meta-analysis14 which reinforces that there is an increase in blood volume during pregnancy and this increase is very significant. By measuring this volume, a low percentage increase suggests complications like pregnancy induced hypertension, pre-eclampsia and foetal growth restriction highlighting why increased blood volume is essential.