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Summary. Fetal haemopoietic cells continually circulate and migrate into tissues, and thus may have specialized homing capabilities. In this study we investigated the in vitro features of haemopoietic cells in fetal blood and liver which are relevant to homing and engraftment. Fetal cells were examined for long-term culture-initiating cell (LTC-IC) and progenitor content, adhesion molecule expression, cell cycle behaviour and transendothelial migratory activity. The LTC-IC content of fetal CD34 + cells is similar to that of CD34 + cells from cord and adult mobilized blood. In contrast to adult and cord blood CD34 + cells, fetal CD34 + cells were actively cycling (11á0 1á7% and 28 1á1% of fetal blood and liver CD34 + cells, respectively, in S+G 2 M, P < 0á001, compared with cord and adult cells). The striking ®nding was that fetal haemopoietic cells (both LTC-ICs and committed progenitors) displayed signi®cantly higher levels of migration across endothelium (P < 0á05 compared with cord, P < 0á01 compared with adult blood and bone marrow CD34 + cells), which were further increased by chemokines and growth factors. The superior migratory activity of fetal haemopoietic cells may underlie a more ef®cient homing ability, in keeping with their physiological role.

The relationship between adverse perinatal outcomes in women with false positive biochemical screening test for Down syndrome was investigated in a retrospective case-controlled study. A cohort of 4000 women who booked for routine antenatal care and opted for biochemical screening over a 22 month period was obtained. The pregnancy outcome data of 272 women with a false positive screening test for Down syndrome (risk >1 in 250) at 15-18 weeks of gestation (study group) were compared with data from 272 age and gestation matched controls with a negative Down syndrome screening test from the same population. The frequency of normal and adverse perinatal outcomes, including pre-eclampsia, isolated intrauterine growth restriction, spontaneous preterm labour and stillbirth was recorded. The incidence of adverse pregnancy outcomes was 11.9% in the study group and 8.6% in the control group. The estimated odds ratio of an abnormal outcome in the study group was 1.41 (95% CI-0.790, 2.55). The observed difference between proportion was 0.0324 (95% CI-0.022, 0.083; p=0.40). These data identify no evidence for a strong association between a false positive Down syndrome screening test result and subsequent adverse perinatal outcomes in the general population.