St. Thomas Aquinas gives a classic pro-life argument.

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Super Sent. lib. 4 d. 32 q. 1 a. 1 ad 4 (=Summa suppl. q. 64 a. 1 ad 4):

uxor etiam viro leproso tenetur reddere debitum [...] et quamvis proles generetur infirma, tamen melius est ei sic esse quam penitus non esse.
 
a wife is bound to pay the debt even to a leprous husband [...] And though the child begotten of them be diseased, it is better to be thus than not at all.

Elige vitam.

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Another pro-life / anti-contraceptive motif is in St. Thomas's commentary on Matt. 19:18, where he explains why ☧ omitted mentioning the First Tablet's commandments (1st-4th, ∵ he know the rich young man already loved God), but mentions the 5th-8th, and explains why they are ordered that way:

non homicidium facies, quod est contra vitam in actu; non adulterabis, quod est contra vitam in potentia
Thou shalt do no murder, which is opposed to actual life: Thou shalt not commit adultery, which is opposed to life in potency

(Interestingly, ☧ didn't mention 9th or 10th, and so the rich young man doesn't lie in v. 20 when, although he committed the sin of covetousness, he says "All these have I kept from my youth".)

Existence (which in God ≡ His divine essence!) is sacred! St. Thomas's pithy summary of the 6th commandment reminded me of that.

Elige vitam semper.

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Casti Connubii §66:

66. What is asserted in favor of the social and eugenic "indication" may and must be accepted, provided lawful and upright methods are employed within the proper limits; but to wish to put forward reasons based upon them for the killing of the innocent is unthinkable and contrary to the divine precept promulgated in the words of the Apostle: Evil is not to be done that good may come of it. [Rom., III, 8.]

St. Thomas gives a eugenic argument in Summa suppl. q. 64 a. 3 co. (=Super Sent. lib. 4 d. 32 q. 1 a. 2 qc. 2 co.):
It is "a moral precept" not "to approach to a menstruous woman" "on account of the harm that frequently results to the offspring from such intercourse".

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harm that frequently results to the offspring from such intercourse".

Is this scientifically verified or was it simply a common belief of the 12th-13th centuries?

The Catholic OB/GYN Thomas W. Hilgers (inventor of NaProTECHNOLOGY) notes that, surprisingly, the most accurate method of determining the day of conception is if the woman daily charts her cycle based upon cervical mucus indicators, etc. (Creighton model / Billings Ovulation Method). The mainstream gynecology industry just assumes ovulation occurs on cycle day 14, LH ovulation test kits are worthless, and HCG (for pregnancy test) isn't high enough concentration until at least a week later. (Accurate timing of this is essential for NaPro's "Targeted Hormone Evaluation and Treatment" (cf. ch. 10 of The NaProTECHNOLOGY Revolution.) The second hurdle to such an experiment would be getting a large enough sample of women, because only a tiny fraction (probably <1%) would actually conceive outside the fertile window.

I don't think anyone doubts menstrual fluid is toxic (e.g., toxic shock syndrome is a risk of tampon usage). Else why does the body get rid of it?

cf. Pope Innocent III's On the Misery of the Human Condition 1.4:

But notice what food the fetus is fed in the womb: with menstrual blood of course, which ceases in the woman after conception so that with it the fetus is fed inside the woman. The blood is said to be so detestable and unclean that "on contact with it crops do not germinate, orchards wither, plants die, trees drop their fruit; if dogs eat of it, they are transported into madness." Conceived fetuses contract the defect of the seed, so that lepers and elephantiacs are born from this corruption.

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Here's another pro-life quote (I-II q. 66 a. 4 "Whether justice is the chief of the moral virtues?" co.):

Maximum autem in his quae ad hominem pertinent, est vita, a qua omnia alia dependent.
in things touching man, the chief of all is life, on which all other things depend.

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Commenting on 1 Tim. 5:14 ("the younger [widows often] should marry, bear children"):

Bear children, and not kill them secretly by abortion

i.e., choosing life is a means of sanctification for them.

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Is it not you [Manichaeans] who used to urge us to observe, to the extent that it was possible, the time when a woman after her menstruation is likely to conceive, and to abstain from intercourse at that time for fear that a soul might become entangled in flesh?

—St. Augustine (388AD), De moribus Ecclesiae catholicae et de moribus Manichaeorum II, 18

[Kephapaulos]

Take that NFP!

"Nihil sub sole novum, nec valet quisquam dicere : Ecce hoc recens est : jam enim praecessit in saeculis quae fuerunt ante nos."

("Nothing under the sun is new, neither is any man able to say: Behold this is new: for it hath already gone before in the ages that were before us.")

(Ecclesiastes 1:10)

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Summa Theologica I-II q. 94 a. 2 co.:

whatever is a means of preserving human life, and of warding off its obstacles, belongs to the natural law.

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Summa Theologica II-II q. 64 a. 6 co.:

it is in no way lawful to slay the innocent.
nullo modo licet occidere innocentem.

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NaProTECHNOLOGY

Catholic Teaching on IVF - Questions with Father #41 - Fr. Palko