Threats & protective factors for infant health - teratogens. Screening for newborn health: atypical development - premature or low birthweight infant. About 120-150 males conceived for every 100 girls. At birth, 106 boys to 100 girl live births. Boys increase frequency of: c-sections needed at birth, developmental disorders. So sperms containing y-chromosomes are very fast and can swim much faster to the egg, but they aren"t very hard. So if you time intercourse exactly when the women is ovulating, you have a much higher chance (like 90%) of conceiving a boy. The x-sperm are slower, but are much hardier. So if the couple is having sex a few days before the women ovulates, x-sperms are the ones left alive when the egg is released, likely to conceive a girl. From the moment of conception onwards, boys are much more likely to die than are girls. Start of placenta: arms, legs, face, organs, muscles all develop.