Lactation or breastfeeding prevents the menstrual or ovarian cycle for the first few months after childbirth if the frequency of sucking is about 8-10 times per 24 hour period.
This effect varies according to the individual but the general rule is – the more frequently you feed (eg every couple of hours) the more likely you will have no period while breastfeeding.
The suppression of ovulation during lactation is thought to be caused by decreased production of FSH and LH due to the signals being sent from the nipple to the brain during breastfeeding.
The return of the menstrual cycle seems to be controlled by how often you feed, and whether you are still feeding through the night.
Periods can return at anytime. For example, women who bottle feed might get their periods returning very soon after birth and those women still breastfeeding regularly (especially if through the night) may not see their bleed till night feeds cease.
It all varies and its important to realise there are no strict rules in this regard.
Breastfeeding by itself is a unreliable form of contraception as ovulation will usually occur before the return of the menstrual cycle. Therefore you could be ovulating and not know it (depending on how familiar you are at spotting your bodies signs of ovulation while nursing).
So waiting for your first period before thinking you are fertile again is dangerous if you don’t wish to be pregnant.