This type of issue comes up quite regularly on Mumsnet.
I've posted the following advice before under different names
Have you been googling anything vaguely related or might you have anything in your browsing history related to pregnancy. It's not just based on things you have searched for. Google monitors what your 'interests' are and then displays what it thinks are relevant adverts just for you. The are called 'interest based' adverts.
To stop it happening again I would firstly clear out my cookies and search history. (I'm not actually sure this is necessary but it doesn't do any harm
)
If you go to GOOGLE ad preferences you can opt out of interest based adverts on Google AND across the web.
I have done this. It means I just get generic ads instead. I have to opt out again, when I delete my cookies.
If you use an ipad or iphone with ios7 you can also TURN ON 'Limit Ad Tracking'
Settings>>Privacy>>>>Advertising>>. Toggle Limit Ad Tracking On
You can also turn off location based ad's if you use ios7 (this will stop Apple tracking where you are and using the info to send you spammy ads ad's from nearby businesses.
Settings>>>>Privacy>>>>location services >>>>(scroll right down to the bottom of the page)) system services >>> then TOGGLE OFF 'Location-based iAds.
HERE is more info about how to opt out of interest based advertising on other search engines, Facebook and twitter etc.