How Does Natural Pregnancy Occur?

By BSc, MSc (embryologist) and BA, MA (fertility counselor).
Last Update: 08/26/2015

Pregnancy occurs after engaging into unsafe sexual intercourse. This means that no contraception is used, either barrier methods such as condoms, or chemical contraceptives like the birth control pill.

In a young and healthy couple, every sexual intercourse taking place during the fertile days of a woman has an average of 20-30% success rate for getting pregnant, and after one year, this rate reaches 85%.

This is the reason why resorting to a fertility clinic is not advisable before a whole year trying to conceive naturally has passed since there are chances for a sudden, natural childbirth to occur.

Requirements to have a natural childbirth

The man should have at least 15 million sperm per milliliter, and at least 32% of them should be able to swim adequately, with normal sperm motility. Having a natural pregnancy is still possible with lower values of seminal fluid, although it is more complicated.

There should be no obstructions in the female reproductive tract since any blockage may impede the free passage of sperm into the egg, which can be found at the Fallopian tubes after ovulation.

In addition, it is important for the woman to have regular cycles so that the ovary is able to release an egg each month. Otherwise pregnancy wouldn’t occur. Besides, the women’s age also affects her fertility: the older the ovaries get, the poorer the quality of her eggs becomes. This leads to trouble conceiving and greater chances of having a miscarriage.

Journey to pregnancy step by step

You will find step by step the three essential procedures that must take place in order to achieve pregnancy in the following sections.

Vaginal penetration

This is a key step for natural pregnancy. The penis is inserted into the vagina. During ejaculation, the sperm move from the testicles to the penis. Then, they mingle with the seminal fluid produced by the “accessory glands” connected to the urethra. Finally, sperm is inserted inside the vagina.

Passage of sperm from the vagina to the uterus

Once inside the vagina, spermatozoa must overcome the unfavorable pH conditions in which they find themselves in order to be able to enter the cervix and start this way their journey towards the Fallopian tubes, where they will finally meet the egg.

If the woman is not on her fertile days, the sperm will find it harder to cross the cervix by themselves. Conversely, this process gets easier when the woman is ovulating.

Then, the sperm is placed in the internal openings of the cervix and then start the process of hyperactivation, which will allow them to fertilize the oocyte. After that, they must swim quickly to make their way towards the Fallopian tubes.


If the couple engages into sexual intercourse during the woman’s fertile days, then a mature egg (ovum) ready to be fertilized by sperm will be available in the Fallopian tubes.

While sperm can live up to 72 hours within the female reproductive system, an egg can survive for up to 48 hours once released from the ovary. This means the period days of a woman last for up to 4 or 5 days. Thus, if you engage into sexual intercourse during these days, chances of getting pregnant are higher.

How many sperms are necessary to fertilize an egg?

Although it is an only sperm that enters the egg and gives raise to an embryo, it is necessary to have many more of them into the ejaculate, because they must go through a passage full of obstacles until they reach the egg: vagina, cervix, uterus, Fallopian tubes, and ampulla, which is the part of the tube where fertilization takes place.

At best, only a few hundred thousand out of millions of ejaculated sperm reach the correct Fallopian tube. It is only once near the egg when they will be able to break the layers that surround it thanks to a micro bomb they have on their head called acrosome.

A large amount of sperm is necessary to cross the egg coat, even if they don’t fertilize it, until the one that’s not the faster, but rather the most opportune, finds the way open and connects to the internal layer. There it will enter the cell and fertilize the egg.


As mentioned earlier, the sperm union with the egg (i.e. fertilization, giving rise to an embryo) must take place so that a woman gets pregnant.

The ovum is located inside a cyst-like structures known as ovarian follicles. Each follicle has an immature egg that will develop eventually till it is mature. It is at this point that the egg will be ready to break through the surface of the ovary.

The process by which the follicle ruptures and an ovum is released is called ovulation, a montly process in which both ovaries do equal work. Contrary to popular belief, ovulation can rotate from one ovary to the other, but it does not mean you always alternate each month. In fact, although some women of reproductive age do ovulate on the same side, alternation between the right and the left ovary occurs generally on a 50-50 basis.

Where there is no ovulation, the sperm won’t find an egg at its arrival into the Fallopian tubes and, therefore, pregnancy won’t occur. This is the reason why, when TTC, timing of sexual intercourse in relation to ovulation will double your chances of pregnancy.

Which are my fertile days?

If you get your periods right on schedule, calculate 14 days before the next time your period is supposed to start and this will approximately be the day when you will begin ovulating. Ovulation tests calculate the peak of your ovulatory hormone, i.e. LH hormone (luteinizing hormone), which increases just before ovulation. Ovulation tests have become the most accurate way to find out which are your fertile days.

In case you have irregular periods or occasionally you miss a period, turning to a gynecologist is highly advisable if you are trying to conceive because, if there's no egg within the Fallopian tubes ready to be fertilized, natural pregnancy won't take place.

FAQs from users

Can I become pregnant while on my period?

By Laura Garrido BSc, MSc (embryologist).

Even if ovulation normally takes place 14 days before the period, sometimes women ovulate at the same time of the previous period, and for this reason some pregnancies happen while you're on your period.

IVF pregnancy vs. natural pregnancy, is there any difference?

By Blanca Paraíso MD, PhD, MSc (gynecologist).

There exist no differences between a natural pregnancy and a pregnancy that has been achieved using reproductive technologies such as IVF. After the embryo transfer, fetal development will be the exactly the same.

Neither the risk of malformations nor the risk of miscarriage increase when using a fertility treatment. Some studies have discovered a slightly higher risk of preterm birth or low birth weight. Anyway, these complications do not seem to be directly linked to the use of fertility treatments, but with the cause of infertility: women aged 40 or older, uterine anomalies and other pathologies... This type of pregnancies must be monitored very closely.

Is it possible for a woman to be pregnant and still have periods?

By Laura Garrido BSc, MSc (embryologist).

The period is the release of the endometrium of the previous cycle, and takes place when there was no embryo implantation. Thus, usually there is no pregnancy if you are on your period.

If I had three normal periods and the fourth is delayed, could I have fallen pregnant before the first one?

By Laura Garrido BSc, MSc (embryologist).

Absolutely not. Every menstruation means a cycle without embryo; therefore, if you had unsafe sexual intercourse before a series of regular periods, pregnancy is unlikely the reason for the delay.

Can I get pregnant without penetration?

By Laura Garrido BSc, MSc (embryologist).

No. Neither through masturbation, neither through anal penetration, nor through vaginal frictions. Vaginal penetration is necessary for a pregnancy to take place. "And without ejaculation?" There are different opinions about the presence of sperm into the pre-seminal fluid, still the famous "pull-out method" is responsible for several unwanted pregnancies. Thus, if penetration takes place, there's no contraceptive efficacy.

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 Laura Garrido
Laura Garrido
BSc, MSc
Bachelor's Degree in Biotechnology from the Pablo de Olavide University (UPO) of Seville, Spain. Master's Degree in Biotechnology of Human Assisted Reproduction from the University of Valencia (UV) and the Valencian Infertility Institute (IVI). Experience at IVF, andrology, and general analysis laboratories. Embryologist specialized in Assisted Reproduction. More information about Laura Garrido
Adapted into english by:
 Sandra Fernández
Sandra Fernández
Fertility Counselor
Bachelor of Arts in Translation and Interpreting (English, Spanish, Catalan, German) from the University of Valencia (UV) and Heriot-Watt University, Riccarton Campus (Edinburgh, UK). Postgraduate Course in Legal Translation from the University of Valencia. Specialist in Medical Translation, with several years of experience in the field of Assisted Reproduction. More information about Sandra Fernández

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