I've heard that a woman should separate from men after a divorce for 3 months, so that there isn't a question as to who the father is. If this didn't occur, but a single period elapsed, would this also be sufficient bidieved?
There is a wealth of material in this issue in the mamzerus responsa beginning from around the period of the Noda B'Yehuda. Basically, the Talmud states clearly that a woman may continue to have her period until 3 months into her pregnancy (Niddah 7b and 8b):
מעוברת משיודע עוברה ... וכמה הכרת העובר סומכוס אומר משום רבי מאיר שלשה חדשים
A pregnant woman [may assume she will not be getting her period] from the time the pregnancy is known... and how much is that? Sumchos says in the name of Rebbi Meir three months
In the times of the early Acharonim, it became a topic of discussion that experience showed that women stopped getting their period before this time, possibly due to a change in female physiology.
The Noda B'Yehuda in Even HaEzer 69, who was asked whether a woman who had gotten her period since her husband had passed away and then had a child could be assumed to have had the child out of wedlock, argued that it was still common for women to bleed during the first three months (and even afterward in some cases), and that the reports to the contrary were not to be relied on to defame a woman in good standing.
Rebbi Akiva Eiger (born 1761), in a responsum (#128) regarding a woman who was now pregnant and whose husband had been out of town since the last time she had her period, wrote that even though it is uncommon for women to bleed once they conceive, it is still a plausible enough occurence that one cannot rule out the possibility that she was pregnant from before she had her period (and the child is therefore not a mamzer).
Chasam Sofer (born 1762), in responsum YD 169, makes the assertion that bleeding is reduced upon conception but still continues until three months.
Rav Vozner (born 1913) (Shiurei Shevet HaLevi 189:33 #2) assumes that one cannot be lenient due to this reality and obviate concerns of vesst. One would assume that would apply to the prohibition of havchana (abstaining for three months after divorce) as well.
There are a few opinions with which I am familiar that entertain taking the "new reality" into account. The Sidrei Tahara to YD 194:2 cites the opinion of the Avodas HaGershuni (born 1618) and the Shvus Yaakov (born 1661) that the age of a miscarried fetus can be calculated from the date of the most recent period, implying that the pregnancy can be assumed to not have begun before then, and the qualification of the Talmud was merely defining the point at which one reliably knows a woman is pregnant. [Teshuva Me'ahava (1:70) implies that he in principle agrees to this.] Sidrei Tahara himself rejects this approach, as do the Chavas Da'as and Chasam Sofer.
Be'er Moshe 1:48:3 refers to a lenient opinion which is based on the assumption that the three months was only in the times of the Talmud when women continued bleeding until three months. However, he leaves the matter unresolved, and is only discussing vesstos, not havchana.
So the overwhelming consensus, certainly of the later authorities, is that, despite the fact that we see that women do not necessarily have their period during the first three months of pregnancy, the halacha does not change to reflect that fact.
I don't think that such a question is for a blog. the accepted psak is that as Double AA has written, however just for the sake of something different here's a Chidah (Chomas Anach Shmuel 2 11 4) ותבא אליו וישכב עמה וכו' פי' ואין כאן חשש אסור משום הבחנה כי היא מתקדשת מטומאתה מנדתה וכיון שהיה לה ארח כנשים ודאי אינה מעוברת וגם היא פנויה כי היה לה גט מבעלה דכל היוצא למלחמת בית דדוד גט כריתות כותב לאשתו מפרשים