There is an authoritative Vedic literature called Piṇḍa-siddhi in which the scientific understanding of pregnancy is very nicely described. It is stated that when the male secretion enters the menstrual flux in the uterus in two successive drops, the mother develops two embryos in her womb, and she brings forth twins in a reverse order to that in which they were first conceived; the child conceived first is born later, and the one conceived later is brought forth first. The first child conceived in the womb lives behind the second child, so when birth takes place the second child appears first, and the first child appears second. In this case it is understood that Hiraṇyākṣa, the second child conceived, was delivered first, whereas Hiraṇyakaśipu, the child who was behind him, having been conceived first, was born second.
(Srimad Bhagavatam-----3:17:18----purport).

Kardama Muni was a great soul, and therefore he was under the spiritual energy, which means that he was directly connected with the Supreme Lord. The symptom of this is Kṛṣṇa consciousness, constant engagement in devotional service. This was known to Devahūti, yet she was anxious to have a son by bodily union with the sage. She reminded her husband of his promise to her parents: "I will remain only until the time of Devahūti's pregnancy." She reminded him that for a chaste woman to have a child by a great personality is most glorious. She wanted to be pregnant, and she prayed for that. The word strī means "expansion." By bodily union of the husband and wife their qualities are expanded: children born of good parents are expansions of the parents' personal qualifications. Both Kardama Muni and Devahūti were spiritually enlightened; therefore she desired from the beginning that first she be pregnant and then she be empowered with the achievement of God's grace and love of God.
(Srimad Bhagavatam-----3:23:10----purport).

Everything needed is mentioned in the scriptures—what sort of house and decorations there should be, what sort of dress the wife should have, how she should be decorated with ointments, scents and other attractive features, etc. With these requisites fulfilled, the husband will be attracted by her beauty, and a favorable mental situation will be created. The mental situation at the time of sex life may then be transferred into the womb of the wife, and good children can come out of that pregnancy.
(Srimad Bhagavatam-----3:23:11----purport).

Since the child depends completely on the assimilated foodstuff of the mother, during pregnancy there are restrictions on the food taken by the mother. Too much salt, chili, onion and similar food is forbidden for the pregnant mother because the child's body is too delicate and new for him to tolerate such pungent food. Restrictions and precautions to be taken by the pregnant mother, as enunciated in the smṛti scriptures of Vedic literature, are very useful.
(Srimad Bhagavatam-----3:31:5----purport).

The garbhādhāna ceremony before sexual intercourse was compulsory for persons in the higher grades of society, and it is very scientific. Other processes recommended in the Vedic literature during pregnancy are also very important. To take care of the child is the primary duty of the parents because if such care is taken, society will be filled with good population to maintain the peace and prosperity of the society, country and human race.
(Srimad Bhagavatam-----3:31:5----purport).

It is said that when a woman is having labor pains she promises that she will never again become pregnant and suffer from such a severely painful condition. Similarly, when one is undergoing some surgical operation he promises that he will never again act in such a way as to become diseased and have to undergo medical surgery, or when one falls into danger, he promises that he will never again make the same mistake. Similarly, the living entity, when put into a hellish condition of life, prays to the Lord that he will never again commit sinful activities and have to be put into the womb for repeated birth and death. In the hellish condition within the womb, the living entity is very much afraid of being born again, but when he is out of the womb, when he is in full life and good health, he forgets everything and commits again and again the same sins for which he was put into that horrible condition of existence.
(Srimad Bhagavatam-----3:31:11----purport).

Similarly, according to religious injunctions a man is restricted to enjoy sex only once in a month, after the menstrual period of the wife, and if the wife is pregnant, he is not allowed sex life at all. That is the law for human beings. A man is allowed to keep more than one wife because he cannot enjoy sex when the wife is pregnant. If he wants to enjoy sex at such a time, he may go to another wife who is not pregnant. These are laws mentioned in the Manu-saṁhitā and other scriptures.
(Srimad Bhagavatam-----4:27:5----purport).

According to the Vedic scriptures the contraceptive method should be restraint in sex life. It is not that one should indulge in unrestricted sex life and avoid children by using some method to check pregnancy. If a man is in good consciousness, he consults with his religious wife, and as a result of this consultation, with intelligence, one advances in his ability to estimate the value of life. In other words, if one is fortunate enough to have a good, conscientious wife, he can decide by mutual consultation that human life is meant for advancing in Kṛṣṇa consciousness and not for begetting a large number of children.
(Srimad Bhagavatam-----4:27:6----purport).

A person enjoys illicit sex, and due to his blind, lusty desire, he thinks that no one can see him, but this illicit sex is thoroughly observed by the agents of the Supreme Personality of Godhead. Therefore the person is punished in so many ways, presently in Kali-yuga there are many pregnancies due to illicit sex, and sometimes abortions ensue. These sinful activities are witnessed by the agents of the Supreme Personality of Godhead, and a man and woman who create such a situation are punished in the future by the stringent laws of material nature (daivī hy eṣā guṇa-mayī mama māyā duratyayā.
(Srimad Bhagavatam-----5:14:9----purport).

The Supreme Personality of Godhead does not allow anyone to act against the stringent laws of material nature; therefore illicit sex is punished life after life. Illicit sex creates pregnancies, and these unwanted pregnancies lead to abortion. Those involved become implicated in these sins, so much so that they are punished in the same way the next life. Thus in the next life they also enter the womb of a mother and are killed in the same way.
(Srimad Bhagavatam-----5:14:9----purport).

Another point in this verse is that in all eight of these heavenly varṣas, although men and women enjoy sex pleasure, there is no pregnancy. Pregnancy takes place only in lower-grade life. For example, animals like dogs and hogs become pregnant twice a year, and each time they beget at least half a dozen offspring. Even lower species of life such as snakes give birth to hundreds of young at one time. This verse informs us that in grades of life higher than ours, pregnancy occurs once in a lifetime. People still have sex life, but there is no pregnancy. In the spiritual world, people are not very attracted to sex life, due to their exalted devotional attitude. Practically speaking, there is no sex life in the spiritual world, but even if sometimes it does occur, there is no pregnancy at all. On the planet earth, however, human beings do become pregnant, although the tendency is to avoid having children. In this sinful age of Kali, people have even taken to the process of killing the child in the womb. This is the most degraded practice; it can only perpetuate the miserable material conditions of those who perform it.
(Srimad Bhagavatam-----5:17:12----purport).