The First 72 Hours of Pregnancy

Pregnancy in itself is a journey, and most women don’t realise that the journey has already started before they take an early pregnancy test and receive a positive result.

Medically, pregnancy is dated from the first day of the last menstrual period; however, this article focuses specifically on the first 72 hours after fertilisation (conception).

The pregnancy journey starts quietly and insidiously, even before a missed period.

The first three days after conception are when your body goes through changes that, although they may not be externally visible, are actually setting the course for your body to have a healthy pregnancy.

In this blog, we will explore the first 72 hours of pregnancy, what happens during the time between fertilisation and the first steps of cell division, how your body reacts to the changes, and why everything is so significant for future implantation and the growth of the pregnancy.

How does pregnancy start in the body?

Pregnancy starts with a sperm meeting an egg and fertilising it. This usually happens within 24 hours of ovulation and mostly happens in the fallopian tubes. The genetic makeup of the sperm and egg fuse to form a zygote.

A Zygote is the first step in forming a baby. At this stage, the genetic makeup to determine the sex of the baby has been created, and unique biological traits have been formed. The pregnancy process has started despite all pregnancy tests showing negative results.

The First 24 hours after fertilisation

Within the first 24 hours, the fertilised egg (zygote) begins its journey through the fallopian tube toward the uterus, a process that typically takes about 3–5 days to complete.

The female body has not begun to produce detectable pregnancy hormones at this point which means the female body will not experience any changes. The uterus can maintain its hormonal state but implantation has not yet taken place at this point.

24 to 48 Hours: Rapid Cell Division Begins

Starting from 24 hours, the zygote begins to divide rapidly. This process is called Cleavage involves repeated cell divisions, progressing from two cells to four, eight, and beyond.

 It continues to double after set periods of time, creating a ball of cells, referred to as a Morula. This large ball of cells is still the same size as the original singular zygote, as no overall size is gained in the process.  

While all of this division is happening, the Morula is simultaneously moving towards the Uterus. This is a critical phase because any abnormalities in cell division may affect future implantation.

48 to 72 Hours: Formation of the Blastocyst

By 48 to 72 hours, the embryo is typically at the 4–8 cell stage and is still travelling through the fallopian tube. It has not yet reached the uterus at this stage.

The morula stage usually forms around Day 4, and the blastocyst typically develops around Day 5–6 after fertilisation.

It is worth noting that implantation does not occur during the first 72 hours. Implantation generally begins around 6–7 days after fertilisation, once the blastocyst reaches the uterus and attaches to the uterine lining.

Why Are the First 72 Hours So Important?

During the first 72 hours, there are no visible symptoms, but the early cell divisions and genetic organisation are crucial in determining whether the embryo will continue developing normally.

Healthy chromosomal alignment and proper cell division during this phase are essential for successful implantation later.

That is why early embryo development is closely observed in medical procedures like IVF, where specialists monitor cell division patterns to assess embryo quality.

Can Lifestyle Factors Affect These Early Hours?

Lifestyle factors do affect the very beginnings of pregnancy. Stress, smoking, and bad diets can have an overall effect on the quality of the egg, the sperm, and overall embryo development.

Stress and bad diets during the preconception and early stages of pregnancy make it harder for a baby to form, and work against a developing baby as good stress management and following a doctor's guidance create a healthy environment for a developing baby.

When Can Pregnancy Be Detected?

Pregnancy tests do not work for the first 72 hours of pregnancy and for good reason. Pregnancy tests work by detecting an increase in the hormone called hCG. This hormone is only present after the embryo has implanted, which occurs after 6-10 days after the egg has been fertilised.

Blood tests can provide an answer more quickly than urine tests, but they still do not work for the first couple of days. This is completely normal and does not mean that there is an issue with the pregnancy.

The First 72 Hours in IVF Pregnancies

In IVF pregnancies, the first 72 hours are closely monitored in the laboratory after fertilisation. Embryos are typically assessed on Day 2 and Day 3 for proper cell division, and further grading often occurs at the blastocyst stage on Day 5.

This helps doctors identify embryos with healthy development patterns and improves the chances of successful implantation.

When Should You Consult a Fertility Specialist?

If a couple has been trying to get pregnant, and it isn’t happening, and there have been repeated miscarriages, or there is a known history of fertility issues, working with a fertility specialist can help figure out what the reasons are. They can evaluate it early and begin working on personalised treatment.

It may also help to have pre-conception counselling. Couples also planning to get pregnant may want to do it to help improve their health before they get pregnant.

Conclusion

The first 72 hours of pregnancy are a powerful but quiet phase marked by rapid logging of cell divisions. While there are no symptoms during this period, early changes this time are vital in determining how the rest of the pregnancy will go.

For individuals or couples looking for supervision during the initial stages of pregnancy, conception or fertility treatments, SCI IVF Hospital, which is recognised as one of the best IVF centres in India, provides the best advanced reproductive care and personalised support in conjunction with veteran specialists in the field to nurture a healthy pregnancy.

FAQs – The First 72 Hours of Pregnancy

1. What happens in the first 72 hours of pregnancy?

During the first 72 hours after fertilisation, the sperm and egg combine to form a zygote. The zygote begins rapid cell division while travelling through the fallopian tube toward the uterus. Implantation does not occur during this time. Although there are no visible symptoms, these early cell divisions are crucial for healthy embryo development.

2. Can pregnancy symptoms show in 2 days?

No, most women do not experience noticeable pregnancy symptoms within the first 2 days after conception. Hormones like hCG are not produced until after implantation, which usually occurs 6–10 days after fertilisation. Any symptoms felt within 48 hours are unlikely to be pregnancy-related.

3. What are the immediate signs of conceiving?

There are usually no immediate signs of conception. The body does not yet produce detectable pregnancy hormones in the first few days. Some women may notice very mild changes such as slight fatigue or discharge, but these are not reliable indicators of pregnancy.

4. Can pregnancy be detected in the first 72 hours?

No. Pregnancy tests work by detecting hCG hormone, which is released only after implantation. Since implantation occurs about a week after fertilisation, pregnancy cannot be detected in the first 72 hours.

5. When do early pregnancy symptoms actually begin?

Early pregnancy symptoms typically begin after implantation, around 1–2 weeks after conception. Common early signs include missed period, breast tenderness, nausea, fatigue, and mild cramping.