Adult settlers will start colonization, but eventually children will be born on Mars, the next generation of a growing population of Mars colonists. Either the progeny of the first settlers continue the colonization process or, to avoid inbreeding depression, embryos with a maximum of genetic diversity are brought up (inspired by The Cascade).
Effects of gravity on growth
Experiments in zero gravity show that animal embryos do not develop properly in zero-g. Mars has 0.38 gravity, so, even if the said child was not deformed, that child would develop and grow in 0.38 gravity. So, the child may be extremely healthy by Mars standards, but severely weak by Earth standards.