By Zia H Shah MD

Mammalian Pregnancy and the Placenta: Biology and Evolution

Mammalian pregnancy exhibits remarkable complexity, centered on the placenta – a transient but vital organ that forms after embryo implantation. Placentation is the development of a placenta and its associated structures; its function is to transfer nutrients, respiratory gases, water, and metabolic waste between mother and embryo​ en.wikipedia.org​ pmc.ncbi.nlm.nih.gov. In eutherian (placental) mammals, the placenta originates when the embryo implants in the uterine wall, growing a network of chorionic villi that invade the maternal endometrium to establish intimate vascular connections ​pmc.ncbi.nlm.nih.gov​ en.wikipedia.org.

Mammalian placentas vary widely in structure and invasiveness. They are classified by the number of maternal tissue layers separating maternal and fetal blood. Epitheliochorial placentas (e.g. ruminants, horses, whales, dugongs) have an intact maternal epithelium between mother and fetus; endotheliochorial placentas (e.g. dogs, cats) lose the maternal uterine epithelium so fetal chorion lies adjacent to maternal endothelium; and hemochorial placentas (e.g. humans, rodents, rabbits, guinea pigs) bring fetal chorion in direct contact with maternal blood​ en.wikipedia.org​. Hemochorial placentation allows more efficient nutrient and gas exchange but requires advanced maternal immune tolerance to avoid fetal rejection ​en.wikipedia.org​ pmc.ncbi.nlm.nih.gov. In humans, placental development begins about 7–8 days after fertilization: chorionic villi on the embryonic side (chorion frondosum) proliferate and attach to the maternal decidua basalis to form the definitive placenta​ en.wikipedia.org​.

Placental Roles: The placenta’s fundamental roles can be summarized as follows:

• Nutrient and gas exchange: It mediates oxygen uptake and carbon dioxide/waste removal, delivering maternal nutrients (glucose, amino acids, lipids) to the fetus ​pmc.ncbi.nlm.nih.gov ​en.wikipedia.org. In early pregnancy the placenta is the sole organ of exchange, sustaining rapid fetal growth.
• Immune modulation and protection: The placenta actively regulates the maternal immune system to tolerate the semi-allogenic fetus and protects the embryo/fetus from pathogens. Trophoblast cells express pattern-recognition receptors (e.g. Toll-like receptors) and secrete factors that attract and “educate” maternal immune cells, establishing an immune milieu favorable to pregnancy​. As one immunology review notes, “the placenta is a key mediator of the fetal/maternal interaction by providing signals that regulate the function of the maternal immune system” ​ pmc.ncbi.nlm.nih.gov. The syncytiotrophoblast (outer placental layer) resists many infections, further shielding the fetus​ pmc.ncbi.nlm.nih.gov.
• Endocrine (hormonal) support: The placenta acts as an endocrine organ producing steroid and peptide hormones that regulate maternal physiology and fetal development ​pmc.ncbi.nlm.nih.gov. For example, by term the human placenta produces on the order of 200–300 mg progesterone daily ​pmc.ncbi.nlm.nih.gov, maintaining the uterine lining and suppressing maternal immune attack on fetal antigens​ pmc.ncbi.nlm.nih.gov. Placental trophoblasts also produce estrogens (from fetal precursors), human chorionic gonadotropin (hCG) to sustain the corpus luteum, and human placental lactogen and other proteins that modulate maternal metabolism and growth ​pmc.ncbi.nlm.nih.gov​.
• Waste disposal: Metabolic wastes (urea, CO₂) from the fetus diffuse back to maternal blood for excretion.

These functions demonstrate the placenta’s centrality in supporting embryonic and fetal development.

Species-Specific Placental Adaptations

Placental form and function show species-specific adaptations. For instance, the extent of placental invasiveness correlates with prenatal investment: animals with epitheliochorial placentas (e.g. cows, sheep, horses) have three maternal tissue layers protecting the fetal membranes, while hemochorial placentas (humans, mice, rabbits) are highly invasive ​en.wikipedia.org. This difference affects fetal nutrition and immunity. In hemochorial species, maternal IgG antibodies cross the placenta to the fetus, whereas in epitheliochorial species IgG transfer in utero is minimal. As a result, ungulates must deliver antibody-rich colostrum at birth to confer passive immunity, whereas humans and rodents rely more on prenatal IgG transfer​ pmc.ncbi.nlm.nih.gov. Indeed, one review notes that in species with exclusive postnatal immunity transfer (all ungulates), colostrum is especially rich in IgG; in species with significant prenatal IgG transfer (humans, rabbits) colostrum emphasizes IgA for gut protection ​pmc.ncbi.nlm.nih.gov.

Placental adaptations also involve morphology. For example, pigs have a diffuse, epitheliochorial placenta with specialized areolae for absorption; cows and sheep have multiple discrete cotyledons; and humans/primates have a discoid, hemochorial placenta. Marsupials (e.g. kangaroos) form a transient, yolk-sac–based (choriovitelline) placenta for their altricial young, whereas eutherians generally develop a chorioallantoic placenta. Such diversity reflects evolutionary trade-offs between gestation length, litter size, and maternal physiology​ en.wikipedia.org ​pmc.ncbi.nlm.nih.gov.

Placental Immunology and Endocrinology

The placenta is also an immunological and hormonal interface. Besides the aforementioned immune tolerogenic signaling​ pmc.ncbi.nlm.nih.gov, it produces hormones that influence maternal and fetal physiology. For example:

• Progesterone – largely placenta-derived after the first trimester – is crucial for pregnancy maintenance and actively down-regulates maternal cytotoxic immunity ​ pmc.ncbi.nlm.nih.gov. The human placenta synthesizes ~250 mg/day by term​ pmc.ncbi.nlm.nih.gov.
• Human chorionic gonadotropin (hCG) – secreted early by trophoblast – rescues the corpus luteum to sustain progesterone production​.
• Placental lactogen and Growth Hormones – modify maternal metabolism (e.g. insulin resistance) and promote fetal growth​.
• Other peptides – placental relaxin, corticotropin-releasing hormone (CRH), and others, regulate uterine blood flow, parturition timing, and fetal development​.

Molecularly, the placenta relies on both maternal and fetal precursors. Human trophoblast lacks certain steroidogenic enzymes (e.g. 17α-hydroxylase), so it converts fetal dehydroepiandrosterone into estrogens and borrows corticosteroid precursors from the fetus​ pmc.ncbi.nlm.nih.gov. Thus the “fetoplacental unit” is tightly integrated biochemically. Overall, the placenta exemplifies a multi-functional organ: it transfers nutrients and oxygen, removes wastes, shields and tolerates the fetus immunologically, and secretes hormones to regulate the pregnancy​.

Evolutionary Context

Placental reproduction evolved once in mammals (excluding egg-laying monotremes) but has parallels in other vertebrates. Remarkably, placentas have independently evolved over 100 times among vertebrates (notably many lizard and shark lineages)​ en.wikipedia.org. In mammals, the divergence of Eutheria (placental mammals) and Metatheria (marsupials) reflects alternative reproductive strategies: eutherians rely on a longer gestation with a complex chorioallantoic placenta, whereas marsupials give birth early to undeveloped young after a brief yolk-sac–based placental phase. Across placental mammals, evolutionary innovations include modified placental hormones (e.g. prolactin variants in ruminants), unique villous structures (e.g. hemophagous regions in bats), and specialized nutrient transporters. These adaptions align the placental function to species’ ecological niches, litter size, and offspring development.

Embryonic Development and Resurrection in the Qur’an

The Qur’an repeatedly uses the stages of human embryogenesis as a sign of God’s creative power and, by extension, evidence of His ability to resurrect the dead. Three key passages frame this theme:

• Surah Al-Mu’minūn (23:12–16): “We created man from an essence of clay, then We placed him as a drop of fluid in a safe place, then We made that drop into a clinging form, and We made that form into a lump of flesh, and We made that lump into bones, and We clothed those bones with flesh, and later We made him into other forms ––glory be to God, the best of creators!––then you will die and then, on the Day of Resurrection, you will be raised up again.” ​
• Surah Al-Ḥajj (22:5–6): “People, [remember,] if you doubt the Resurrection, that We created you from dust, then a drop of fluid, then a clinging form, then a lump of flesh, both shaped and unshaped: We mean to make Our power clear to you. Whatever We choose We cause to remain in the womb for an appointed time, then We bring you forth as infants and then you grow and reach maturity. Some die young and some are left to live on to such an age that they forget all they once knew. You sometimes see the earth lifeless, yet when We send down water it stirs and swells and produces every kind of joyous growth: this is because God is the Truth; He brings the dead back to life; He has power over everything.” ​
• Surah Al-Qiyāmah (75:36–40): “Does man think he will be left alone? Was he not just a drop of spilt-out sperm, which became a clinging form, which God shaped in due proportion, fashioning from it the two sexes, male and female? Does He who can do this not have the power to bring the dead back to life?”

In each passage the sequential image is similar: human life begins from inert origins (clay or a sperm-drop), progresses through stages as a “clinging clot” and “chewed lump” of tissue, and finally emerges as a fully formed living being. Classical Arabic tafsīr (exegesis) identifies technical terms: nutfah (sperm drop), ʿalaqah (often translated “clinging clot” or “leech-like mass”), and mudghah (“chewed lump”), with bones then being formed and covered by flesh ​legacy.quran.com.

Scholarly Interpretations

Classical commentators and modern scholars emphasize that these Quranic descriptions serve a theological purpose. They do not claim to be a biological manual, but rather use the well-known growth of the human embryo as an argument for resurrection. For example, one classical tafsīr notes that when disbelievers deride God’s power to “revive decayed bones” (as in Qur’an 36:78), God responds by recalling His act of creating humans from dust and bodily fluids ​islamweb.net. In other words, who created the human from nothing but clay and a drop of fluid can surely recreate the human after death. This point is made explicit in the last verse of Surah 75: “Does He not have the power to bring back the dead to life?”​

Modern commentators echo this. Syed Abul Aʿlā Maududi writes that the “whole act of creation, starting from the emission of a sperm-drop till its development into a perfect man,” is a clear manifestation of Allah’s power and wisdom – so much so that one who reflects cannot deny life-after-death ​islamicstudies.info. Maududi observes:

“This is an argument for the possibility of life-afterdeath. …the people who believe that the whole act of creation…is only a manifestation of the power and wisdom of Allah, …cannot refuse to admit that the God Who brings about man in the world also has the power to bring the same man into being once again.”​ islamicstudies.info.

Classical tafsīr collections similarly enumerate the stages. For example, Mufti Shafi’s Al-Maʿārif explains that 23:12–14 traces seven stages: from “extract of clay” (or “essence of earth,” i.e. Adam’s creation) to nutfah, ʿalaqah, mudghah, then bones, then flesh, and finally the soul being “blown into” the body ​quran.com. Commentators like Ibn ʿAbbās, Mujāhid and others interpret the final phrase “developed him into another creation” (23:14) as the moment God breathes the nafs al-ammah (the created “biological soul” or spirit) into the formed embryo​ ​quran.com. This, too, marks a transformation: the first six stages deal with material substances, but the seventh is a special “other creation” involving God’s spirit​.

Other classical scholars add detail. For instance, Tafsīr al-Jalālayn and Al-Ḥāfiẓ ibn Kathīr note that “ʿalaqah” linguistically can mean a clot or something that clings (like a leech), reflecting the embryo’s appearance. The term mudghah (“chewed lump”) is also evocative of the somite-stage embryo. The sequence emphasizes progression from elemental to complex form. As one leading tafsīr remarks, the Qur’an uses thumma (“then”) and fa (“and then”) to signal stages that naturally follow, underscoring continuous development, before the final “soul-breathing”​ quran.com.

Biological Development as Theological Proof

Both classical and modern tafsīr stress that these verses are signs (āyāt) of God’s creative ability. The implicit logic is: “If God can create this intricate process once, surely He can do so again after death.” This is made explicit by combining verses 23:14 and 75:40: “So blessed is Allah, the best of creators” and “Does He not have the power to bring back the dead to life?” The juxtaposition of embryology with resurrection is not an incidental detail but a deliberate argument. As Maududi points out, even a skeptic who acknowledges that life begins from a “drop of sperm” must concede that the same Creator could resurrect that life. Similarly, the Qur’an compares a barren earth revived by rain (22:5–7) to the human womb – both serve as analogies that illustrate divine power over life and death.

In summary, the Qur’anic verses frame human development as a proof of God’s omnipotence. Classical and modern scholars agree that the described stages – from clay or fluid to fully formed infant – are meant to inspire reflection on the Afterlife. By drawing on observable embryological processes, the text invites readers to extrapolate: just as God initiated life from inert matter, He is surely capable of recreating the dead for judgment and recompense ​islamicstudies.info.

Thus, scientific insights into placenta and embryology enrich our understanding of both natural biology and Qur’anic symbolism. The placenta’s wondrous functions – mediating life between mother and child – parallel the Qur’an’s depiction of life’s origin, underscoring a harmonious view: biological complexity and scriptural reflection both point to a designed, purposeful creation capable of renewal beyond death​ en.wikipedia.org​ islamicstudies.info.

Having examined all the above quoted verses and detailed biology of pregnancy and embryology, can we be bold enough in our reading of the last verse of the Surah Al Qiyamah, ‘Has not such a One the power to bring the dead to life?‘ and conclude that every scientific detail of mammalian evolution, pregnancy, placental and embryology is proof enough for our Afterlife?