This dissertation, "Markers of Down Syndrome and Fetal Growth Profile in Pregnancies Conceived With Assisted Reproduction" by Pui-wah, Hui, 許佩華, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Assisted reproduction technology is increasingly used for treatment of couples with subfertility. These women are usually of more advanced maternal age and carry a higher risk of fetal Down syndrome. Results from early publications showed that biochemical markers for screening of fetal Down syndrome in the second trimester were different between pregnancies from in vitro fertilization (IVF) and natural conception. This could potentially increase the false positive rate and result in unnecessary invasive diagnostic procedures. Questions were raised as to whether the alterations were related to ovarian stimulation. This laid the fundamentals of a series of studies presented in this thesis with an aim to address the variations in the concentrations of markers of fetal Down syndrome and the fetal growth profile of pregnancies conceived following different assisted reproduction treatments. Studies were conducted on maternal serum and amniotic fluid alpha fetoprotein (AFP) and human chorionic gonadotrophin (hCG) in the second trimester in pregnancies conceived by assisted reproduction. A reduced level of AFP in maternal serum in pregnancies with fresh embryos together with an elevated level of hCG in both maternal serum and amniotic fluid in pregnancies with frozen thawed embryos were found. This pioneer piece of data showing the raised hCG in frozen thawed embryo pregnancies with unstimulated treatment cycles spoke against the ovarian driven hypothesis, but suggested placental dysfunction be a possible underlying pathophysiology. For markers adopted in the first trimester, the level of pregnancy associated protein A (PAPP-A) was significantly reduced in pregnancies from assisted reproduction. The data on free βhCG was heterogeneous. Apart from biochemical markers, the nuchal translucency was also increased in these singleton pregnancies but not in dichorionic twins. As the direction of deviations of these markers in unaffected pregnancies from assisted reproduction resembled those observed in pregnancies affected by Down syndrome, appropriate adjustment was necessary to reduce the false positive rate for these women. Altered biochemical markers, notably a low PAPP-A level, were also associated with adverse obstetric outcomes. The changes observed in pregnancies from assisted reproduction might be a manifestation of an intrinsic placental insufficiency or fetal developmental delay. A longitudinal study was performed to examine the intrauterine fetal growth profile in these pregnancies. The rate of increment in the mean sac size, which could represent an adaptive compensatory mechanism, was significantly greater in pregnancies from assisted reproduction compared to natural conception. We concluded that pregnancies conceived after assisted reproduction technology were different from pregnancies from natural conception in terms of the concentrations of biochemical and ultrasound markers of Down syndrome. Due to the wide variation in treatment protocols and patients' background demographics, the exact underlying pathophysiology might be difficult to be explored. Couples undergoing assisted reproduction treatment should be counseled on the increased risk of adverse pregnancy course and perinatal outcome. DOI: 10.5353/th_b5361008 Subjects: Prenatal diagnosisDown syndrome