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Abnormal fetal growth is closely related to adverse short-term outcomes as well as negative long-term health consequences. Fetal growth restriction is a common pregnancy complication, and for the detection of fetuses small for gestational age, a reliable reference is vital. Increasing evidence supports that mode of conception has implications for growth. Fresh embryo transfer (ET) is associated with small for gestational age, and frozen ET with large for gestational age. The overall aim of this thesis was to describe optimal fetal growth, as well as pre-and postnatal growth after ART, trying to isolate the direct effects of ART from related factors.

Study I was a prospective multicentre study aiming to construct updated population-based references for fetal growth for the Swedish population from a cohort of 583 low-risk pregnancies. Comparisons were also made with other relevant growth charts.

In Study II, a population-based retrospective cohort study including 10 970 fresh ET, 6520 frozen ET, and 178 518 natural conception (NC) pregnancies, fetal growth after ART was explored with longitudinal statistics. Study III, a prospective longitudinal multicentre study of 82 fresh ET, 175 frozen ET and the 583 NC from Study I as reference, had similar aims. Fetuses after ART in general tended to be larger than NC in early pregnancy and thereafter growing at a slower rate than NC. This was more pronounced after fresh ET and at term mean fetal weight was lower than NC. Frozen ET remained heavier than NC in Study II but in Study III there were no significant differences.

Study IV was a population-based retrospective cohort study of 517 fresh ET, 284 frozen ET and  17 214 NC, examining growth from birth to five years of age. At birth, children after fresh ET were smaller, and those after frozen ET were larger than NC. Most differences were attenuated by 18 months of age.

The new references will in future studies be evaluated for the intended population. Our results support that there is a direct effect of ART on fetal growth. Although differences diminish with age, the differences in growth are known risk factors for future cardiometabolic disease.