We have a legal requirement to accommodate a 1 yr mat leave here, which can be taken by one parent or shared by the two. We've had men and women take it, in whole or in part. Because we're now a large enough office to handle being short a body or two, we work around it. Is it disruptive to business? Absolutely- but way less disruptive than when a key staff member quits or has an accident and needs to spend months in hospital, or any number of other things that can and do happen- because unlike those eventualities, you get plenty of notice to PLAN around the person's mat leave. I have no idea how a much smaller office could manage to handle it though- it would be very tough to lose your sole engineer or even your #3, for a whole year. Tt's a legal requirement to keep that person's job open for them, so you figure it out and work around it.
Is the risk of service interruption due to mat leave, a deterrent to hiring young women for engineering positions? I'd say it absolutely must be, especially in small offices, in ANY responsible position irrespective of profession. Call it unfair if you like, but it is impossible to imagine employers divorcing themselves completely from that kind of discrimination- ever. Hold on a minute- I just imagined a scenario! OK, so it will remain so until they start requiring a matching, mandatory (and paid) paternity leave such that all persons of reproductive age represent exactly the same risk of having to be off the job for a year with each kid they have, irrespective of sex. Is that likely to happen? I'd give it a diminishingly small probability.
As to the proportions of male and female engineers: in chemical engineering at the university we recruit co-op students from, the female to male ratio was around 30% 25 years ago when I graduated and is now over 40%. Proportions in civil and systems design engineering are similar. At proportions like that, I see no need to be concerned about the fact that some of the other subdiscipolines- electrical/computer and mechanical- having somewhat lower proportions of women. On the job, we find vastly more variation in performance between individuals than we do between the sexes.