{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,30]],"date-time":"2025-10-30T17:28:55Z","timestamp":1761845335830,"version":"3.40.5"},"reference-count":34,"publisher":"Wiley","issue":"9","license":[{"start":{"date-parts":[[2021,2,12]],"date-time":"2021-02-12T00:00:00Z","timestamp":1613088000000},"content-version":"vor","delay-in-days":0,"URL":"http:\/\/creativecommons.org\/licenses\/by-nc-nd\/4.0\/"}],"content-domain":{"domain":["onlinelibrary.wiley.com"],"crossmark-restriction":true},"short-container-title":["Birth Defects Research"],"published-print":{"date-parts":[[2021,5,15]]},"abstract":"<jats:title>Abstract<\/jats:title><jats:sec><jats:title>Background<\/jats:title><jats:p>Clinical observation suggests that acardiac twinning occurs only in the first trimester. In part, this contradicts our previous analysis (part IV) of Benirschke's concept that unequal embryonic splitting causes unequal embryo\/fetal blood volumes and pressures. Our aim is to explain why acardiac onset is restricted to the first trimester.<\/jats:p><\/jats:sec><jats:sec><jats:title>Methods<\/jats:title><jats:p>We applied the vascular resistance scheme of two fetuses connected by arterio\u2010arterial (AA) and veno\u2010venous (VV) anastomoses, the small VV resistance approximated as zero. The smaller twin has volume fraction <jats:italic>\u03b1<\/jats:italic> &lt;\u20091 of the assumed normal larger twin, and has only access to fraction <jats:italic>X<\/jats:italic> &lt;\u20091 of its placenta; the larger twin's larger mean arterial pressure accesses the remaining fraction. Before 13\u2009weeks, embryos have a much smaller vascular resistance than placentas. After 13\u2009weeks, when maternal blood provides oxygen, smaller twins can increase their vascular volume by hypoxemia\u2010mediated neovascularization. Estimated AA radii at 40\u2009weeks, <jats:styled-content><jats:italic>r<\/jats:italic><jats:sub><jats:italic>AA<\/jats:italic><\/jats:sub>(40)<\/jats:styled-content>, are 0.5\u20131.3\u2009mm.<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>Embryos with <jats:italic>\u03b1<\/jats:italic> &lt;\u20090.33 unlikely survive 13\u2009weeks and acardiac twinning occurs under appropriate conditions (AA\u2010VV, small placenta). Acardiac body perfusion occurs because of a much smaller vascular resistance than the placenta. When <jats:italic>\u03b1<\/jats:italic> &gt;\u20090.33 and <jats:styled-content><jats:italic>r<\/jats:italic><jats:sub><jats:italic>AA<\/jats:italic><\/jats:sub>(40)<\/jats:styled-content>=1.3\u2009mm, modeled survival is &gt;32\u2009weeks.<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusion<\/jats:title><jats:p>Before 13\u2009weeks, embryos with <jats:italic>\u03b1<\/jats:italic> &lt;\u20090.33 cannot survive and may result in the onset of acardia. Beyond 13\u2009weeks, fetuses with <jats:italic>\u03b1<\/jats:italic> \u2265\u20090.33 survive because <jats:styled-content><jats:italic>r<\/jats:italic><jats:sub><jats:italic>AA<\/jats:italic><\/jats:sub>(40)<\/jats:styled-content> is too small for acardiac onset. Following fetal demise, exsanguination from the live twin increases its blood volume and, we assumed also, its vascular resistance. Perfusion then occurs through the lower resistance placenta.<\/jats:p><\/jats:sec>","DOI":"10.1002\/bdr2.1882","type":"journal-article","created":{"date-parts":[[2021,2,14]],"date-time":"2021-02-14T16:02:59Z","timestamp":1613318579000},"page":"687-695","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":5,"title":["Acardiac twin pregnancies part <scp>VI<\/scp>: <i>Why<\/i> does acardiac twinning occur only in the first trimester?"],"prefix":"10.1002","volume":"113","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-3747-6219","authenticated-orcid":false,"given":"Martin J. C.","family":"van Gemert","sequence":"first","affiliation":[{"name":"Department of Biomedical Engineering &amp; Physics Amsterdam University Medical Centers  Amsterdam The Netherlands"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Michael G.","family":"Ross","sequence":"additional","affiliation":[{"name":"Department of Obstetrics and Gynecology Harbor UCLA Medical Center  Torrance California USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6648-8967","authenticated-orcid":false,"given":"Jeroen P. H. M.","family":"van den Wijngaard","sequence":"additional","affiliation":[{"name":"Department of Clinical Chemistry, Hematology and Immunology Diakonessenhuis  Utrecht The Netherlands"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Peter G. J.","family":"Nikkels","sequence":"additional","affiliation":[{"name":"Department of Pathology Wilhelmina Children's Hospital, University Medical Center  Utrecht The Netherlands"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"311","published-online":{"date-parts":[[2021,2,12]]},"reference":[{"key":"e_1_2_7_2_1","doi-asserted-by":"publisher","DOI":"10.3109\/01443615.2011.647129"},{"key":"e_1_2_7_3_1","doi-asserted-by":"publisher","DOI":"10.1111\/j.1471-0528.2004.00273.x"},{"key":"e_1_2_7_4_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.placenta.2009.08.009"},{"volume-title":"Pathology of the human placenta","year":"2006","author":"Benirschke K.","key":"e_1_2_7_5_1"},{"key":"e_1_2_7_6_1","doi-asserted-by":"publisher","DOI":"10.1126\/science.7196086"},{"key":"e_1_2_7_7_1","doi-asserted-by":"publisher","DOI":"10.1016\/0029-7844(96)00254-2"},{"key":"e_1_2_7_8_1","doi-asserted-by":"publisher","DOI":"10.1111\/j.1471-0528.1902.tb15905.x"},{"key":"e_1_2_7_9_1","doi-asserted-by":"publisher","DOI":"10.1196\/annals.1389.023"},{"key":"e_1_2_7_10_1","doi-asserted-by":"publisher","DOI":"10.1016\/S0002-9378(00)70233-X"},{"key":"e_1_2_7_11_1","doi-asserted-by":"publisher","DOI":"10.1016\/S0046-8177(98)90213-1"},{"issue":"1","key":"e_1_2_7_12_1","first-page":"1","article-title":"Arteries of the chorion of pregnant women with arterial hypertension","volume":"3","author":"Gansburgskiy A. 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